Method and device for transmitting and receiving tracking reference signal in wireless communication system supporting unlicensed band

ABSTRACT

Disclosed are a TRS receiving method comprising: receiving, from a base station, information indicating that a TRS is transmitted in a subframe; receiving the TRS transmitted in the subframe; and performing tracking by using the TRS, and a terminal performing, in the subframe to which the TRS is transmitted, the demodulation of data or not transmitting an acknowledgement/negative ACK (ACK/NACK) for the data.

TECHNICAL FIELD

Following description relates to a wireless communication systemsupporting an unlicensed band, and more particularly, to a method oftransmitting and receiving a tracking reference signal, a method ofperforming RRM (radio resource measurement), and an apparatus therefor.

BACKGROUND ART

Wireless access systems have been widely deployed to provide varioustypes of communication services such as voice or data. In general, awireless access system is a multiple access system that may supportcommunication of multiple users by sharing available system resources(e.g., a bandwidth, transmission power, etc.). For example, multipleaccess systems include a Code Division Multiple Access (CDMA) system, aFrequency Division Multiple Access (FDMA) system, a Time DivisionMultiple Access (TDMA) system, an Orthogonal Frequency Division MultipleAccess (OFDMA) system, a Single Carrier Frequency Division MultipleAccess (SC-FDMA) system, and a multi carrier frequency division multipleaccess (MC-FDMA) system.

DISCLOSURE OF THE INVENTION Technical Tasks

An object of the present invention is to provide a method of efficientlytransmitting and receiving data in a wireless communication systemsupporting an unlicensed band.

Another object of the present invention is to sufficiently provide atracking reference signal for performing tracking to a user equipmentprior to downlink data transmission.

The other object of the present invention is to make a user equipmentperform RRM measurement and report a result of the RRM measurement to abase station in a situation that a sample for performing the RRMmeasurement is not sufficient.

The technical problems solved by the present invention are not limitedto the above technical problems and other technical problems which arenot described herein will become apparent to those skilled in the artfrom the following description.

Technical Solution

To achieve these and other advantages and in accordance with the purposeof the present invention, as embodied and broadly described, a method ofreceiving a TRS (tracking reference signal), which is received by a userequipment in a wireless communication system supporting an unlicensedband, includes the steps of receiving, from a base station, informationindicating that a TRS is transmitted in a subframe, receiving the TRStransmitted in the subframe, and performing tracking using the TRS. Inthis case, the user equipment does not demodulate data or does nottransmit ACK/NACK (acknowledgment/negative ACK) in the subframe in whichthe TRS is transmitted.

The subframe in which the TRS is transmitted may correspond to asubframe other than a subframe in which a DRS (discovery referencesignal) is transmitted.

The TRS can be composed of at least one selected from the groupconsisting of a DRS, a DMRS (demodulation reference signal), and adownlink transmission preamble.

The information indicating that the TRS is transmitted can includeUE-specifically transmitted DCI (downlink control information) or anRNTI (radio network temporary identifier) commonly assigned to userequipments connected with the base station.

The tracking is performed using TRSs equal to or greater than theprescribed number of TRSs satisfying a tracking requirement configuredby the base station and can be performed using both a TRS receivedwithin a time section configured by the base station and a TRS receivedin the subframe.

If the user equipment determines that a tracking requirement forperforming tracking is not satisfied, the method can further include thestep of requesting TRS transmission to the base station.

If a tracking requirement is not satisfied, the subframe can include asecondary DMTC (discovery measurement timing configuration) configuredwith a period shorter than a period of a primary DMTC.

To further achieve these and other advantages and in accordance with thepurpose of the present invention, according to a different embodiment, auser equipment receiving a TRS (tracking reference signal) in a wirelesscommunication system supporting an unlicensed band includes atransmitter, a receiver, and a processor configured to operate in amanner of being connected with the transmitter and the receiver, theprocessor configured to receive, from a base station, informationindicating that a TRS is transmitted in a subframe, the processorconfigured to receive the TRS transmitted in the subframe, the processorconfigured to perform tracking using the TRS. In this case, the userequipment demodulates data or does not transmit ACK/NACK(acknowledgment/negative ACK) in the subframe in which the TRS istransmitted.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory and areintended to provide further explanation of the invention as claimed.

Advantageous Effects

As is apparent from the above description, the embodiments of thepresent invention have the following effects.

First of all, it is able to efficiently transmit and receive data in awireless communication system supporting an unlicensed band.

Second, since it is able to easily satisfy a tracking requirement forestimating a property of a radio channel, it is able to smoothlytransmit downlink data.

Third, it is able to make a user equipment report a measurement resultto a base station according to a channel status even in a situation thata sample for performing RRM measurement is not sufficient.

The effects of the present invention are not limited to theabove-described effects and other effects which are not described hereinmay be derived by those skilled in the art from the followingdescription of the embodiments of the present invention. That is,effects which are not intended by the present invention may be derivedby those skilled in the art from the embodiments of the presentinvention.

DESCRIPTION OF DRAWINGS

The effects of the present invention are not limited to theabove-described effects and other effects which are not described hereinmay be derived by those skilled in the art from the followingdescription of the embodiments of the present invention. That is,effects which are not intended by the present invention may be derivedby those skilled in the art from the embodiments of the presentinvention.

FIG. 1 is a diagram illustrating physical channels and a signaltransmission method using the physical channels;

FIG. 2 is a diagram illustrating exemplary radio frame structures;

FIG. 3 is a diagram illustrating an exemplary resource grid for theduration of a downlink slot;

FIG. 4 is a diagram illustrating an exemplary structure of an uplinksubframe;

FIG. 5 is a diagram illustrating an exemplary structure of a downlinksubframe;

FIG. 6 is a view illustrating Physical Uplink Control Channel (PUCCH)formats 1a and 1b in a normal Cyclic Prefix (CP) case, and FIG. 7 is aview illustrating PUCCH formats 1a and 1b in an extended CP case;

FIG. 8 is a view illustrating PUCCH format 2/2a/2b in the normal CPcase, and FIG. 9 is a view illustrating PUCCH format 2/2a/2b in theextended CP case;

FIG. 10 is a view illustrating Acknowledgment/Negative Acknowledgment(ACK/NACK) channelization for PUCCH formats 1a an 1b;

FIG. 11 is a view illustrating channelization for a hybrid structure ofPUCCH format 1a/1b and PUCCH format 2/2a/2b in the same PhysicalResource Block (PRB);

FIG. 12 is a view illustrating a PRB allocation method;

FIG. 13 is a view illustrating exemplary Component Carriers (CCs) andexemplary Carrier Aggregation (CA) in a Long Term Evolution-Advanced(LTE-A) system, which are used in embodiments of the present disclosure;

FIG. 14 is a view illustrating a subframe structure based oncross-carrier scheduling in the LTE-A system, which is used inembodiments of the present disclosure;

FIG. 15 is a view illustrating an exemplary configuration of servingcells according to cross-carrier scheduling used in embodiments of thepresent disclosure;

FIG. 16 is a view illustrating an exemplary new PUCCH format based onblock spreading;

FIG. 17 is a view illustrating an exemplary configuration of a ResourceUnit (RB) with time-frequency units;

FIG. 18 is a view illustrating an exemplary method for resourceallocation and retransmission in asynchronous Hybrid Automatic RepeatreQuest (HARQ);

FIG. 19 is a conceptual view illustrating a Coordinated Multi-Point(CoMP) system operating in a CA environment;

FIG. 20 is a view illustrating an exemplary subframe to which UserEquipment (UE)-specific Reference Signals (RSs) (UE-RSs) are allocated,which may be used in embodiments of the present disclosure;

FIG. 21 is a view illustrating an exemplary multiplexing of a legacyPhysical Downlink Channel (PDCCH), a Physical Downlink Shared Channel(PDSCH), and an Enhanced PDCCH (E-PDCCH) in the LTE/LTE-A system;

FIG. 22 is a diagram illustrating an exemplary CA environment supportedin an LTE-Unlicensed (LTE-U) system;

FIG. 23 is a diagram illustrating an exemplary Frame Based Equipment(FBE) operation as one of Listen-Before-Talk (LBT) operations;

FIG. 24 is a block diagram illustrating the FBE operation;

FIG. 25 is a diagram illustrating an exemplary Load Based Equipment(LBE) operation as one of the LBT operations;

FIG. 26 is a diagram for explaining DRS transmission methods supportedby LAA system;

FIG. 27 is a flowchart for explaining CAP and CWA;

FIGS. 28 to 30 are diagrams illustrating a method of transmitting andreceiving a TRS (tracking reference signal) according to a proposedembodiment;

FIG. 31 is a flowchart for a method of performing RRM (radio resourcemeasurement) according to a proposed embodiment;

FIG. 32 is a diagram for configurations of a user equipment and a basestation capable of implementing proposed embodiments.

BEST MODE Mode for Invention

The following embodiments are proposed by combining constituentcomponents and characteristics of the present invention according to apredetermined format. The individual constituent components orcharacteristics should be considered optional factors on the conditionthat there is no additional remark. If required, the individualconstituent components or characteristics may not be combined with othercomponents or characteristics. In addition, some constituent componentsand/or characteristics may be combined to implement the embodiments ofthe present invention. The order of operations to be disclosed in theembodiments of the present invention may be changed. Some components orcharacteristics of any embodiment may also be included in otherembodiments, or may be replaced with those of the other embodiments asnecessary.

In describing the present invention, if it is determined that thedetailed description of a related known function or construction rendersthe scope of the present invention unnecessarily ambiguous, the detaileddescription thereof will be omitted.

In the entire specification, when a certain portion “comprises orincludes” a certain component, this indicates that the other componentsare not excluded and may be further included unless specially describedotherwise. The terms “unit”, “-or/er” and “module” described in thespecification indicate a unit for processing at least one function oroperation, which may be implemented by hardware, software or acombination thereof. The words “a or an”, “one”, “the” and words relatedthereto may be used to include both a singular expression and a pluralexpression unless the context describing the present invention(particularly, the context of the following claims) clearly indicatesotherwise.

In the embodiments of the present disclosure, a description is mainlymade of a data transmission and reception relationship between a BaseStation (BS) and a User Equipment (UE). A BS refers to a terminal nodeof a network, which directly communicates with a UE. A specificoperation described as being performed by the BS may be performed by anupper node of the BS.

Namely, it is apparent that, in a network comprised of a plurality ofnetwork nodes including a BS, various operations performed forcommunication with a UE may be performed by the BS, or network nodesother than the BS. The term ‘BS’ may be replaced with a fixed station, aNode B, an evolved Node B (eNode B or eNB), an Advanced Base Station(ABS), an access point, etc.

In the embodiments of the present disclosure, the term terminal may bereplaced with a UE, a Mobile Station (MS), a Subscriber Station (SS), aMobile Subscriber Station (MSS), a mobile terminal, an Advanced MobileStation (AMS), etc.

A transmission end is a fixed and/or mobile node that provides a dataservice or a voice service and a reception end is a fixed and/or mobilenode that receives a data service or a voice service. Therefore, a UEmay serve as a transmission end and a BS may serve as a reception end,on an UpLink (UL). Likewise, the UE may serve as a reception end and theBS may serve as a transmission end, on a DownLink (DL).

The embodiments of the present disclosure may be supported by standardspecifications disclosed for at least one of wireless access systemsincluding an Institute of Electrical and Electronics Engineers (IEEE)802.xx system, a 3rd Generation Partnership Project (3GPP) system, a3GPP Long Term Evolution (LTE) system, and a 3GPP2 system. Inparticular, the embodiments of the present disclosure may be supportedby the standard specifications, 3GPP TS 36.211, 3GPP TS 36.212, 3GPP TS36.213, 3GPP TS 36.321 and 3GPP TS 36.331. That is, the steps or parts,which are not described to clearly reveal the technical idea of thepresent disclosure, in the embodiments of the present disclosure may beexplained by the above standard specifications. All terms used in theembodiments of the present disclosure may be explained by the standardspecifications.

Reference will now be made in detail to the embodiments of the presentdisclosure with reference to the accompanying drawings. The detaileddescription, which will be given below with reference to theaccompanying drawings, is intended to explain exemplary embodiments ofthe present disclosure, rather than to show the only embodiments thatcan be implemented according to the disclosure.

The following detailed description includes specific terms in order toprovide a thorough understanding of the present disclosure. However, itwill be apparent to those skilled in the art that the specific terms maybe replaced with other terms without departing the technical spirit andscope of the present disclosure.

For example, the term, TxOP may be used interchangeably withtransmission period or Reserved Resource Period (RRP) in the same sense.Further, a Listen-Before-Talk (LBT) procedure may be performed for thesame purpose as a carrier sensing procedure for determining whether achannel state is idle or busy.

Hereinafter, 3GPP LTE/LTE-A systems are explained, which are examples ofwireless access systems.

The embodiments of the present disclosure can be applied to variouswireless access systems such as Code Division Multiple Access (CDMA),Frequency Division Multiple Access (FDMA), Time Division Multiple Access(TDMA), Orthogonal Frequency Division Multiple Access (OFDMA), SingleCarrier Frequency Division Multiple Access (SC-FDMA), etc.

CDMA may be implemented as a radio technology such as UniversalTerrestrial Radio Access (UTRA) or CDMA2000. TDMA may be implemented asa radio technology such as Global System for Mobile communications(GSM)/General packet Radio Service (GPRS)/Enhanced Data Rates for GSMEvolution (EDGE). OFDMA may be implemented as a radio technology such asIEEE 802.11 (Wi-Fi), IEEE 802.16 (WiMAX), IEEE 802.20, Evolved UTRA(E-UTRA), etc.

UTRA is a part of Universal Mobile Telecommunications System (UMTS).3GPP LTE is a part of Evolved UMTS (E-UMTS) using E-UTRA, adopting OFDMAfor DL and SC-FDMA for UL. LTE-Advanced (LTE-A) is an evolution of 3GPPLTE. While the embodiments of the present disclosure are described inthe context of a 3GPP LTE/LTE-A system in order to clarify the technicalfeatures of the present disclosure, the present disclosure is alsoapplicable to an IEEE 802.16e/m system, etc.

1. 3GPP LTE/LTE-A System

In a wireless access system, a UE receives information from an eNB on aDL and transmits information to the eNB on a UL. The informationtransmitted and received between the UE and the eNB includes generaldata information and various types of control information. There aremany physical channels according to the types/usages of informationtransmitted and received between the eNB and the UE.

1.1 Generals of System

FIG. 1 illustrates physical channels and a general signal transmissionmethod using the physical channels, which may be used in embodiments ofthe present disclosure.

When a UE is powered on or enters a new cell, the UE performs initialcell search (S11). The initial cell search involves acquisition ofsynchronization to an eNB. Specifically, the UE synchronizes its timingto the eNB and acquires information such as a cell Identifier (ID) byreceiving a Primary Synchronization Channel (P-SCH) and a SecondarySynchronization Channel (S-SCH) from the eNB.

Then the UE may acquire information broadcast in the cell by receiving aPhysical Broadcast Channel (PBCH) from the eNB.

During the initial cell search, the UE may monitor a DL channel state byreceiving a Downlink Reference Signal (DL RS).

After the initial cell search, the UE may acquire more detailed systeminformation by receiving a Physical Downlink Control Channel (PDCCH) andreceiving a Physical Downlink Shared Channel (PDSCH) based oninformation of the PDCCH (S12).

To complete connection to the eNB, the UE may perform a random accessprocedure with the eNB (S13 to S16). In the random access procedure, theUE may transmit a preamble on a Physical Random Access Channel (PRACH)(S13) and may receive a PDCCH and a PDSCH associated with the PDCCH(S14). In the case of contention-based random access, the UE mayadditionally perform a contention resolution procedure includingtransmission of an additional PRACH (S15) and reception of a PDCCHsignal and a PDSCH signal corresponding to the PDCCH signal (S16).

After the above procedure, the UE may receive a PDCCH and/or a PDSCHfrom the eNB (S17) and transmit a Physical Uplink Shared Channel (PUSCH)and/or a Physical Uplink Control Channel (PUCCH) to the eNB (S18), in ageneral UL/DL signal transmission procedure.

Control information that the UE transmits to the eNB is genericallycalled Uplink Control Information (UCI). The UCI includes a HybridAutomatic Repeat and reQuest Acknowledgement/Negative Acknowledgement(HARQ-ACK/NACK), a Scheduling Request (SR), a Channel Quality Indicator(CQI), a Precoding Matrix Index (PMI), a Rank Indicator (RI), etc.

In the LTE system, UCI is generally transmitted on a PUCCH periodically.However, if control information and traffic data should be transmittedsimultaneously, the control information and traffic data may betransmitted on a PUSCH. In addition, the UCI may be transmittedaperiodically on the PUSCH, upon receipt of a request/command from anetwork.

FIG. 2 illustrates exemplary radio frame structures used in embodimentsof the present disclosure.

FIG. 2(a) illustrates frame structure type 1. Frame structure type 1 isapplicable to both a full Frequency Division Duplex (FDD) system and ahalf FDD system.

One radio frame is 10 ms (Tf=307200·Ts) long, including equal-sized 20slots indexed from 0 to 19. Each slot is 0.5 ms (Tslot=15360·Ts) long.One subframe includes two successive slots. An ith subframe includes2ith and (2i+1)th slots. That is, a radio frame includes 10 subframes. Atime required for transmitting one subframe is defined as a TransmissionTime Interval (TTI). Ts is a sampling time given as Ts=1/(15kHz×2048)=3.2552×10−8 (about 33 ns). One slot includes a plurality ofOrthogonal Frequency Division Multiplexing (OFDM) symbols or SC-FDMAsymbols in the time domain by a plurality of Resource Blocks (RBs) inthe frequency domain.

A slot includes a plurality of OFDM symbols in the time domain. SinceOFDMA is adopted for DL in the 3GPP LTE system, one OFDM symbolrepresents one symbol period. An OFDM symbol may be called an SC-FDMAsymbol or symbol period. An RB is a resource allocation unit including aplurality of contiguous subcarriers in one slot.

In a full FDD system, each of 10 subframes may be used simultaneouslyfor DL transmission and UL transmission during a 10-ms duration. The DLtransmission and the UL transmission are distinguished by frequency. Onthe other hand, a UE cannot perform transmission and receptionsimultaneously in a half FDD system.

The above radio frame structure is purely exemplary. Thus, the number ofsubframes in a radio frame, the number of slots in a subframe, and thenumber of OFDM symbols in a slot may be changed.

FIG. 2(b) illustrates frame structure type 2. Frame structure type 2 isapplied to a Time Division Duplex (TDD) system. One radio frame is 10 ms(Tf=307200·Ts) long, including two half-frames each having a length of 5ms (=153600·Ts) long. Each half-frame includes five subframes each being1 ms (=30720·Ts) long. An ith subframe includes 2ith and (2i+1)th slotseach having a length of 0.5 ms (Tslot=15360·Ts). Ts is a sampling timegiven as Ts=1/(15 kHz×2048)=3.2552×10−8 (about 33 ns).

A type-2 frame includes a special subframe having three fields, DownlinkPilot Time Slot (DwPTS), Guard Period (GP), and Uplink Pilot Time Slot(UpPTS). The DwPTS is used for initial cell search, synchronization, orchannel estimation at a UE, and the UpPTS is used for channel estimationand UL transmission synchronization with a UE at an eNB. The GP is usedto cancel UL interference between a UL and a DL, caused by themulti-path delay of a DL signal.

[Table 1] below lists special subframe configurations (DwPTS/GP/UpPTSlengths).

TABLE 1 Normal cyclic prefix in downlink Extended cyclic prefix indownlink UpPTS upPTS Special Normal Extendend Normal Extended subframecyclic prefix cyclic prefix cyclic prefix cyclic prefix configurationDwPTS in uplink in uplink DwPTS uplink uplink 0  6592 · T_(s) 2192 ·T_(s) 2560 · T_(s)  7680 · T_(s) 2192 · T_(s) 2560 · T_(s) 1 19760 ·T_(s) 20480 · T_(s) 2 21952 · T_(s) 23040 · T_(s) 3 24144 · T_(s) 25600· T_(s) 4 26336 · T_(s)  7680 · T_(s) 4384 · T_(s) 5120 · T_(s) 5  6592· T_(s) 4384 · T_(s) 5120 · T_(s) 20480 · T_(s) 6 19760 · T_(s) 23040 ·T_(s) 7 21952 · T_(s) — — — 8 34144 · T_(s) — — —

FIG. 3 illustrates an exemplary structure of a DL resource grid for theduration of one DL slot, which may be used in embodiments of the presentdisclosure.

Referring to FIG. 3, a DL slot includes a plurality of OFDM symbols inthe time domain. One DL slot includes 7 OFDM symbols in the time domainand an RB includes 12 subcarriers in the frequency domain, to which thepresent disclosure is not limited.

Each element of the resource grid is referred to as a Resource Element(RE). An RB includes 12×7 REs. The number of RBs in a DL slot, NDLdepends on a DL transmission bandwidth. A UL slot may have the samestructure as a DL slot.

FIG. 4 illustrates a structure of a UL subframe which may be used inembodiments of the present disclosure.

Referring to FIG. 4, a UL subframe may be divided into a control regionand a data region in the frequency domain. A PUCCH carrying UCI isallocated to the control region and a PUSCH carrying user data isallocated to the data region. To maintain a single carrier property, aUE does not transmit a PUCCH and a PUSCH simultaneously. A pair of RBsin a subframe are allocated to a PUCCH for a UE. The RBs of the RB pairoccupy different subcarriers in two slots. Thus it is said that the RBpair frequency-hops over a slot boundary.

FIG. 5 illustrates a structure of a DL subframe that may be used inembodiments of the present disclosure.

Referring to FIG. 5, up to three OFDM symbols of a DL subframe, startingfrom OFDM symbol 0 are used as a control region to which controlchannels are allocated and the other OFDM symbols of the DL subframe areused as a data region to which a PDSCH is allocated. DL control channelsdefined for the 3GPP LTE system include a Physical Control FormatIndicator Channel (PCFICH), a PDCCH, and a Physical Hybrid ARQ IndicatorChannel (PHICH).

The PCFICH is transmitted in the first OFDM symbol of a subframe,carrying information about the number of OFDM symbols used fortransmission of control channels (i.e. the size of the control region)in the subframe. The PHICH is a response channel to a UL transmission,delivering an HARQ ACK/NACK signal. Control information carried on thePDCCH is called Downlink Control Information (DCI). The DCI transportsUL resource assignment information, DL resource assignment information,or UL Transmission (Tx) power control commands for a UE group.

1.2 Physical Downlink Control Channel (PDCCH)

1.2.1 PDCCH Overview

The PDCCH may deliver information about resource allocation and atransport format for a Downlink Shared Channel (DL-SCH) (i.e. a DLgrant), information about resource allocation and a transport format foran Uplink Shared Channel (UL-SCH) (i.e. a UL grant), paging informationof a Paging Channel (PCH), system information on the DL-SCH, informationabout resource allocation for a higher-layer control message such as arandom access response transmitted on the PDSCH, a set of Tx powercontrol commands for individual UEs of a UE group, Voice Over InternetProtocol (VoIP) activation indication information, etc.

A plurality of PDCCHs may be transmitted in the control region. A UE maymonitor a plurality of PDCCHs. A PDCCH is transmitted in an aggregate ofone or more consecutive Control Channel Elements (CCEs). A PDCCH made upof one or more consecutive CCEs may be transmitted in the control regionafter subblock interleaving. A CCE is a logical allocation unit used toprovide a PDCCH at a code rate based on the state of a radio channel. ACCE includes a plurality of RE Groups (REGs). The format of a PDCCH andthe number of available bits for the PDCCH are determined according tothe relationship between the number of CCEs and a code rate provided bythe CCEs.

1.2.2 PDCCH Structure

A plurality of PDCCHs for a plurality of UEs may be multiplexed andtransmitted in the control region. A PDCCH is made up of an aggregate ofone or more consecutive CCEs. A CCE is a unit of 9 REGs each REGincluding 4 REs. Four Quadrature Phase Shift Keying (QPSK) symbols aremapped to each REG. REs occupied by RSs are excluded from REGs. That is,the total number of REGs in an OFDM symbol may be changed depending onthe presence or absence of a cell-specific RS. The concept of an REG towhich four REs are mapped is also applicable to other DL controlchannels (e.g. the PCFICH or the PHICH). Let the number of REGs that arenot allocated to the PCFICH or the PHICH be denoted by NREG. Then thenumber of CCEs available to the system is NCCE (=└N_(REG)/9┘) and theCCEs are indexed from 0 to NCCE-1.

To simplify the decoding process of a UE, a PDCCH format including nCCEs may start with a CCE having an index equal to a multiple of n. Thatis, given CCE i, the PDCCH format may start with a CCE satisfying i modn=0.

The eNB may configure a PDCCH with 1, 2, 4, or 8 CCEs. {1, 2, 4, 8} arecalled CCE aggregation levels. The number of CCEs used for transmissionof a PDCCH is determined according to a channel state by the eNB. Forexample, one CCE is sufficient for a PDCCH directed to a UE in a good DLchannel state (a UE near to the eNB). On the other hand, 8 CCEs may berequired for a PDCCH directed to a UE in a poor DL channel state (a UEat a cell edge) in order to ensure sufficient robustness.

[Table 2] below illustrates PDCCH formats. 4 PDCCH formats are supportedaccording to CCE aggregation levels as illustrated in [Table 2].

TABLE 2 PDCCH Number of Number of Number of format CCE (n) REG PDCCHbits 0 1 9 72 1 2 18 144 2 4 36 288 3 8 72 576

A different CCE aggregation level is allocated to each UE because theformat or Modulation and Coding Scheme (MCS) level of controlinformation delivered in a PDCCH for the UE is different. An MCS leveldefines a code rate used for data coding and a modulation order. Anadaptive MCS level is used for link adaptation. In general, three orfour MCS levels may be considered for control channels carrying controlinformation.

Regarding the formats of control information, control informationtransmitted on a PDCCH is called DCI. The configuration of informationin PDCCH payload may be changed depending on the DCI format. The PDCCHpayload is information bits. [Table 3] lists DCI according to DCIformats.

TABLE 3 DCI Format Description Format Resource grants for PUSCHtransmissions (uplink) 0 Format Resource assignments for single codewordPDSCH transmission 1 (transmission modes 1, 2 and 7) Format Compactsignaling of resource assignments for single codeword 1A PDSCH (allmodes) Format Compact resource assignments for PDSCH using rank-1 closed1B loop precoding (mode 6) Format Very compact resource assignments forPDSCH (e.g., 1C paging/broadcast system information) Format Compactresource assignments for PDSCH using multi-user 1D MIMO(mode 5) FormatResource assignments for PDSCH for closed loop MIMO 2 operation (mode 4)Format resource assignments for PDSCH for open loop MIMO operation 2A(mode 3) Format Power control commands for PUCCH and PUSCH with 2-bit/1-3/3A bit power adjustment Format Scheduling of PUSCH in one UL cell withmulti-antenna port 4 transmission mode

Referring to [Table 3], the DCI formats include Format 0 for PUSCHscheduling, Format 1 for single-codeword PDSCH scheduling, Format 1A forcompact single-codeword PDSCH scheduling, Format 1C for very compactDL-SCH scheduling, Format 2 for PDSCH scheduling in a closed-loopspatial multiplexing mode, Format 2A for PDSCH scheduling in anopen-loop spatial multiplexing mode, and Format 3/3A for transmission ofTransmission Power Control (TPC) commands for uplink channels. DCIFormat 1A is available for PDSCH scheduling irrespective of thetransmission mode of a UE.

The length of PDCCH payload may vary with DCI formats. In addition, thetype and length of PDCCH payload may be changed depending on compact ornon-compact scheduling or the transmission mode of a UE.

The transmission mode of a UE may be configured for DL data reception ona PDSCH at the UE. For example, DL data carried on a PDSCH includesscheduled data, a paging message, a random access response, broadcastinformation on a BCCH, etc. for a UE. The DL data of the PDSCH isrelated to a DCI format signaled through a PDCCH. The transmission modemay be configured semi-statically for the UE by higher-layer signaling(e.g. Radio Resource Control (RRC) signaling). The transmission mode maybe classified as single antenna transmission or multi-antennatransmission.

A transmission mode is configured for a UE semi-statically byhigher-layer signaling. For example, multi-antenna transmission schememay include transmit diversity, open-loop or closed-loop spatialmultiplexing, Multi-User Multiple Input Multiple Output (MU-MIMO), orbeamforming. Transmit diversity increases transmission reliability bytransmitting the same data through multiple Tx antennas. Spatialmultiplexing enables high-speed data transmission without increasing asystem bandwidth by simultaneously transmitting different data throughmultiple Tx antennas. Beamforming is a technique of increasing theSignal to Interference plus Noise Ratio (SINR) of a signal by weightingmultiple antennas according to channel states.

A DCI format for a UE depends on the transmission mode of the UE. The UEhas a reference DCI format monitored according to the transmission modeconfigure for the UE. The following 10 transmission modes are availableto UEs:

(1) Transmission mode 1: Single antenna port (port 0);

(2) Transmission mode 2: Transmit diversity;

(3) Transmission mode 3: Open-loop spatial multiplexing when the numberof layer is larger than 1 or Transmit diversity when the rank is 1;

(4) Transmission mode 4: Closed-loop spatial multiplexing;

(5) Transmission mode 5: MU-MIMO;

(6) Transmission mode 6: Closed-loop rank-1 precoding;

(7) Transmission mode 7: Precoding supporting a single layertransmission, which is not based on a codebook (Rel-8);

(8) Transmission mode 8: Precoding supporting up to two layers, whichare not based on a codebook (Rel-9);

(9) Transmission mode 9: Precoding supporting up to eight layers, whichare not based on a codebook (Rel-10); and

(10) Transmission mode 10: Precoding supporting up to eight layers,which are not based on a codebook, used for CoMP (Rel-11).

1.2.3 PDCCH Transmission

The eNB determines a PDCCH format according to DCI that will betransmitted to the UE and adds a Cyclic Redundancy Check (CRC) to thecontrol information. The CRC is masked by a unique Identifier (ID) (e.g.a Radio Network Temporary Identifier (RNTI)) according to the owner orusage of the PDCCH. If the PDCCH is destined for a specific UE, the CRCmay be masked by a unique ID (e.g. a cell-RNTI (C-RNTI)) of the UE. Ifthe PDCCH carries a paging message, the CRC of the PDCCH may be maskedby a paging indicator ID (e.g. a Paging-RNTI (P-RNTI)). If the PDCCHcarries system information, particularly, a System Information Block(SIB), its CRC may be masked by a system information ID (e.g. a SystemInformation RNTI (SI-RNTI)). To indicate that the PDCCH carries a randomaccess response to a random access preamble transmitted by a UE, its CRCmay be masked by a Random Access-RNTI (RA-RNTI).

Then, the eNB generates coded data by channel-encoding the CRC-addedcontrol information. The channel coding may be performed at a code ratecorresponding to an MCS level. The eNB rate-matches the coded dataaccording to a CCE aggregation level allocated to a PDCCH format andgenerates modulation symbols by modulating the coded data. Herein, amodulation order corresponding to the MCS level may be used for themodulation. The CCE aggregation level for the modulation symbols of aPDCCH may be one of 1, 2, 4, and 8. Subsequently, the eNB maps themodulation symbols to physical REs (i.e. CCE to RE mapping).

1.2.4 Blind Decoding (BD)

A plurality of PDCCHs may be transmitted in a subframe. That is, thecontrol region of a subframe includes a plurality of CCEs, CCE 0 to CCENCCE,k-1. NCCE,k is the total number of CCEs in the control region of akth subframe. A UE monitors a plurality of PDCCHs in every subframe.This means that the UE attempts to decode each PDCCH according to amonitored PDCCH format.

The eNB does not provide the UE with information about the position of aPDCCH directed to the UE in an allocated control region of a subframe.Without knowledge of the position, CCE aggregation level, or DCI formatof its PDCCH, the UE searches for its PDCCH by monitoring a set of PDCCHcandidates in the subframe in order to receive a control channel fromthe eNB. This is called blind decoding. Blind decoding is the process ofdemasking a CRC part with a UE ID, checking a CRC error, and determiningwhether a corresponding PDCCH is a control channel directed to a UE bythe UE.

The UE monitors a PDCCH in every subframe to receive data transmitted tothe UE in an active mode. In a Discontinuous Reception (DRX) mode, theUE wakes up in a monitoring interval of every DRX cycle and monitors aPDCCH in a subframe corresponding to the monitoring interval. ThePDCCH-monitored subframe is called a non-DRX subframe.

To receive its PDCCH, the UE should blind-decode all CCEs of the controlregion of the non-DRX subframe. Without knowledge of a transmitted PDCCHformat, the UE should decode all PDCCHs with all possible CCEaggregation levels until the UE succeeds in blind-decoding a PDCCH inevery non-DRX subframe. Since the UE does not know the number of CCEsused for its PDCCH, the UE should attempt detection with all possibleCCE aggregation levels until the UE succeeds in blind decoding of aPDCCH.

In the LTE system, the concept of Search Space (SS) is defined for blinddecoding of a UE. An SS is a set of PDCCH candidates that a UE willmonitor. The SS may have a different size for each PDCCH format. Thereare two types of SSs, Common Search Space (CSS) andUE-specific/Dedicated Search Space (USS).

While all UEs may know the size of a CSS, a USS may be configured foreach individual UE. Accordingly, a UE should monitor both a CSS and aUSS to decode a PDCCH. As a consequence, the UE performs up to 44 blinddecodings in one subframe, except for blind decodings based on differentCRC values (e.g., C-RNTI, P-RNTI, SI-RNTI, and RA-RNTI).

In view of the constraints of an SS, the eNB may not secure CCEresources to transmit PDCCHs to all intended UEs in a given subframe.This situation occurs because the remaining resources except forallocated CCEs may not be included in an SS for a specific UE. Tominimize this obstacle that may continue in the next subframe, aUE-specific hopping sequence may apply to the starting position of aUSS.

[Table 4] illustrates the sizes of CSSs and USSs.

TABLE 4 PDCCH Number of Number of Number of Format CCE (n) candidates inCSS candidates in USS 0 1 — 6 1 2 — 6 2 4 4 2 3 8 2 2

To mitigate the load of the UE caused by the number of blind decodingattempts, the UE does not search for all defined DCI formatssimultaneously. Specifically, the UE always searches for DCI Format 0and DCI Format 1A in a USS. Although DCI Format 0 and DCI Format 1A areof the same size, the UE may distinguish the DCI formats by a flag forformat 0/format 1a differentiation included in a PDCCH. Other DCIformats than DCI Format 0 and DCI Format 1A, such as DCI Format 1, DCIFormat 1B, and DCI Format 2 may be required for the UE.

The UE may search for DCI Format 1A and DCI Format 1C in a CSS. The UEmay also be configured to search for DCI Format 3 or 3A in the CSS.Although DCI Format 3 and DCI Format 3A have the same size as DCI Format0 and DCI Format 1A, the UE may distinguish the DCI formats by a CRCscrambled with an ID other than a UE-specific ID.

An SS S_(k) ^((L)) is a PDCCH candidate set with a CCE aggregation levelLϵ{1, 2, 4, 8} The CCEs of PDCCH candidate set m in the SS may bedetermined by the following equation.

L·{(Y _(k) +m)mod ♦N _(CCE,k) /L┘}+i  [Equation 1]

Herein, M^((L)) is the number of PDCCH candidates with CCE aggregationlevel L to be monitored in the SS, m=0, . . . , M^((L))−1, i is theindex of a CCE in each PDCCH candidate, and i=0, . . . , L−1.k=└n_(s)/2┘ where n_(s) is the index of a slot in a radio frame.

As described before, the UE monitors both the USS and the CSS to decodea PDCCH. The CSS supports PDCCHs with CCE aggregation levels {4, 8} andthe USS supports PDCCHs with CCE aggregation levels {1, 2, 4, 8}. [Table5] illustrates PDCCH candidates monitored by a UE.

TABLE 5 Search space S_(k) ^((L)) Number of PDCCH Type Aggregation levelL Size [in CCEs] candidates M^((L)) UE-specific 1 6 6 2 12 6 4 8 2 8 162 Common 4 16 4 8 16 2

Referring to [Equation 1], for two aggregation levels, L=4 and L=8,Y_(k) is set to 0 in the CSS, whereas Y_(k) is defined by [Equation 2]for aggregation level L in the USS.

Y _(k)=(A·Y _(k))mod D  [Equation 2]

Herein, Y⁻¹=n_(RNTI)≠0, n_(RNTI) indicating an RNTI value. A=39827 andD=65537.

1.3. PUCCH (Physical Uplink Control Channel)

PUCCH may include the following formats to transmit control information.

(1) Format 1: On-Off keying (OOK) modulation, used for SR (SchedulingRequest)

(2) Format 1a & 1b: Used for ACK/NACK transmission

-   -   1) Format 1a: BPSK ACK/NACK for 1 codeword    -   2) Format 1b: QPSK ACK/NACK for 2 codewords

(3) Format 2: QPSK modulation, used for CQI transmission

(4) Format 2a & Format 2b: Used for simultaneous transmission of CQI andACK/NACK

(5) Format 3: Used for multiple ACK/NACK transmission in a carrieraggregation environment

[Table 6] shows a modulation scheme according to PUCCH format and thenumber of bits per subframe. Table 7 shows the number of referencesignals (RS) per slot according to PUCCH format. Table 8 shows SC-FDMAsymbol location of RS (reference signal) according to PUCCH format. InTable 6, PUCCH format 2a and PUCCH format 2b correspond to a case ofnormal cyclic prefix (CP).

TABLE 6 PUCCH format Modulation scheme No. of bits per subframe, Mbit 1N/A N/A 1a BPSK 1 1b QPSK 2 2 QPSK 20 2a QPSK + BPSK 21 2b QPSK + BPSK22 3 QPSK 48

TABLE 7 PUCCH format Normal CP Extended CP 1, 1a, 1b 3 2 2, 3 2 1 2a, 2b2 N/A

TABLE 8 PUCCH SC-FDMA symbol location of RS format Normal CP Extended CP1, 1a, 1b 2, 3, 4 2, 3 2, 3 1, 5 3 2a, 2b 1, 5 N/A

FIG. 6 shows PUCCH formats 1a and 1b in case of a normal cyclic prefix.And, FIG. 7 shows PUCCH formats 1a and 1b in case of an extended cyclicprefix.

According to the PUCCH formats 1a and 1b, control information of thesame content is repeated in a subframe by slot unit. In each UE,ACK/NACK signal is transmitted on a different resource constructed witha different cyclic shift (CS) (frequency domain code) and an orthogonalcover (OC) or orthogonal cover code (OCC) (time domain spreading code)of CG-CAZAC (computer-generated constant amplitude zero autocorrelation) sequence. For instance, the OC includes Walsh/DFTorthogonal code. If the number of CS and the number of OC are 6 and 3,respectively, total 18 UEs may be multiplexed within the same PRB(physical resource block) with reference to a single antenna. Orthogonalsequences w0, w1, w2 and w3 may be applicable to a random time domain(after FFT modulation) or a random frequency domain (before FFTmodulation).

For persistent scheduling with SR, ACK/NACK resource constructed withCS, OC and PRB (physical resource block) may be allocated to a UEthrough RRC (radio resource control. For non-persistent scheduling withdynamic ACK/NACK, the ACK/NACK resource may be implicitly allocated to aUE using a smallest CCE index of PDCCH corresponding to PDSCH.

Length-4 orthogonal sequence (OC) and length-3 orthogonal sequence forPUCCH format 1/1a/1b are shown in Table 9 and Table 10, respectively.

TABLE 9 Sequence index Orthogonal sequences n_(oc) (n_(s)) [w(0) . . .w(N_(SF) ^(PUCCH) − 1)] 0 [+1 +1 +1 +1] 1 [+1 −1 +1 −1] 2 [+1 −1 −1 +1]

TABLE 10 Sequence index Orthogonal sequences n_(oc) (n_(s)) [w(0) . . .w(N_(SF) ^(PUCCH) − 1)] 0 [1 1 1] 1 [1 e^(j2π/3) e^(j4π/3)] 2 [1e^(j4π/3) e^(j2π/3)]

Orthogonal sequence (OC) [w(0) . . . w(N_(RS) ^(PUCCH)−1)] for areference signal in PUCCH format 1/1a/1b is shown in Table 11.

TABLE 11 Sequence index n _(oc) (n_(s)) Normal cyclic prefix Extendedcyclic prefix 0 [1 1 1] [1 1] 1 [1 e^(j2π/3) e^(j4π/3)] [1 −1] 2 [1e^(j4π/3) e^(j2π/3)] N/A

FIG. 8 shows PUCCH format 2/2a/2b in case of a normal cyclic prefix.And, FIG. 9 shows PUCCH format 2/2a/2b in case of an extended cyclicprefix.

Referring to FIG. 8 and FIG. 9, in case of a normal CP, a subframe isconstructed with 10 QPSK data symbols as well as RS symbol. Each QPSKsymbol is spread in a frequency domain by CS and is then mapped to acorresponding SC-FDMA symbol. SC-FDMA symbol level CS hopping may beapplied to randomize inter-cell interference. The RS may be multiplexedby CDM using a cyclic shift. For instance, assuming that the number ofavailable CSs is 12, 12 UEs may be multiplexed in the same PRB. Forinstance, assuming that the number of available CSs is 6, 6 UEs may bemultiplexed in the same PRB. In brief, a plurality of UEs in PUCCHformat 1/1a/1b and PUCCH format 2/2a/2b may be multiplexed by‘CS+OC+PRB’ and ‘CS+PRB’, respectively.

FIG. 10 is a diagram of ACK/NACK channelization for PUCCH formats 1a and1b. In particular, FIG. 10 corresponds to a case of ‘Δ_(shift)^(PUCCH)=2’

FIG. 11 is a diagram of channelization for a hybrid structure of PUCCHformat 1/1a/1b and PUCCH format 2/2a/2b.

Cyclic shift (CS) hopping and orthogonal cover (OC) remapping may beapplicable in a following manner.

(1) Symbol-based cell-specific CS hopping for randomization ofinter-cell interference

(2) Slot level CS/OC remapping

-   -   1) For inter-cell interference randomization    -   2) Slot based access for mapping between ACK/NACK channel and        resource (k)

Meanwhile, resource n_(r) for PUCCH format 1/1a/1b may include thefollowing combinations.

(1) CS (=equal to DFT orthogonal code at symbol level) (n_(cs))

(2) OC (orthogonal cover at slot level) (n_(oc))

(3) Frequency RB (Resource Block) (n_(rb))

If indexes indicating CS, OC and RB are set to n_(cs), n_(oc), n_(rb),respectively, a representative index n_(r) may include n_(cs), n_(oc)and n_(rb). In this case, the n_(r) may meet the condition of‘n_(r)=(n_(cs), n_(oc), n_(rb))’.

The combination of CQI, PMI, RI, CQI and ACK/NACK may be deliveredthrough the PUCCH format 2/2a/2b. And, Reed Muller (RM) channel codingmay be applicable.

For instance, channel coding for UL (uplink) CQI in LTE system may bedescribed as follows. First of all, bitstreams a₀, a₁, a₂, a₃, . . . ,a_(A-1) may be coded using (20, A) RM code. In this case, a_(α) anda_(A-1) indicates MSB (Most Significant Bit) and LSB (Least SignificantBit), respectively. In case of an extended cyclic prefix, maximuminformation bits include 11 bits except a case that QI and ACK/NACK aresimultaneously transmitted. After coding has been performed with 20 bitsusing the RM code, QPSK modulation may be applied. Before the BPSKmodulation, coded bits may be scrambled.

Table 12 shows a basic sequence for (20, A) code.

TABLE 12 i M_(i,0) M_(i,i) M_(i,2) M_(i,3) M_(i,4) M_(i,5) M_(i,6)M_(i,7) M_(i,8) M_(i,9) M_(i,10) M_(i,11) M_(i,12)  0 1 1 0 0 0 0 0 0 00 1 1 0  1 1 1 1 0 0 0 0 0 0 1 1 1 0  2 1 0 0 1 0 0 1 0 1 1 1 1 1  3 1 01 1 0 0 0 0 1 0 1 1 1  4 1 1 1 1 0 0 0 1 0 0 1 1 1  5 1 1 0 0 1 0 1 1 10 1 1 1  6 1 0 1 0 1 0 1 0 1 1 1 1 1  7 1 0 0 1 1 0 0 1 1 0 1 1 1  8 1 10 1 1 0 0 1 0 1 1 1 1  9 1 0 1 1 1 0 1 0 0 1 1 1 1 10 1 0 1 0 0 1 1 1 01 1 1 1 11 1 1 1 0 0 1 1 0 1 0 1 1 1 12 1 0 0 1 0 1 0 1 1 1 1 1 1 13 1 10 1 0 1 0 1 0 1 1 1 1 14 1 0 0 0 1 1 0 1 0 0 1 0 1 15 1 1 0 0 1 1 1 1 01 1 0 1 16 1 1 1 0 1 1 1 0 0 1 0 1 1 17 1 0 0 1 1 1 0 0 1 0 0 1 1 18 1 10 1 1 1 1 1 0 0 0 0 0 19 1 0 0 0 0 1 1 0 0 0 0 0 0

Channel coding bits b₀, b₁, b₂, b₃, . . . , b_(B-1) may be generated by[Equation 31].

$\begin{matrix}{b_{i} = {\sum\limits_{n = 0}^{A - 1}{\left( {a_{n} \cdot M_{i,n}} \right){mod}\; 2}}} & \left\lbrack {{Equation}\mspace{14mu} 3} \right\rbrack\end{matrix}$

In [Equation 3], ‘i=0, 1, 2, . . . , B−1’ is met.

In case of wideband repots, a bandwidth of UCI (uplink controlinformation) field for CQI/PMI can be represented as Tables 8 to 10 inthe following.

[Table 13] shows UCI (Uplink Control Information) field for broadbandreport (single antenna port, transmit diversity) or open loop spatialmultiplexing PDSCH CQI feedback.

TABLE 13 Field Bandwidth Wideband CQI 4

[Table 14] shows UL control information (UCI) field for CQI and PMIfeedback in case of wideband reports (closed loop spatial multiplexingPDSCH transmission).

TABLE 14 Bandwidth 2 antenna ports 4 antenna ports Field rank = 1 rank =2 rank = 1 Rank > 1 Wideband CQI 4 4 4 4 Spatial differential CQI 0 3 03 Precoding Matrix 2 1 4 4 Indication

[Table 15] shows UL control information (UCI) field for RI feedback incase of wideband reports.

TABLE 15 Bit widths 4 antenna ports Field 2 antenna ports Max. 2 layersMax. 4 layers Rank Indication 1 1 2

FIG. 12 is a diagram for PRB allocation. Referring to FIG. 20, PRB maybe usable for PUCCH transmission in a slot n_(s).

2. Carrier Aggregation (CA) Environment

2.1 CA Overview

A 3GPP LTE system (conforming to Rel-8 or Rel-9) (hereinafter, referredto as an LTE system) uses Multi-Carrier Modulation (MCM) in which asingle Component Carrier (CC) is divided into a plurality of bands. Incontrast, a 3GPP LTE-A system (hereinafter, referred to an LTE-A system)may use CA by aggregating one or more CCs to support a broader systembandwidth than the LTE system. The term CA is interchangeably used withcarrier combining, multi-CC environment, or multi-carrier environment.

In the present disclosure, multi-carrier means CA (or carriercombining). Herein, CA covers aggregation of contiguous carriers andaggregation of non-contiguous carriers. The number of aggregated CCs maybe different for a DL and a UL. If the number of DL CCs is equal to thenumber of UL CCs, this is called symmetric aggregation. If the number ofDL CCs is different from the number of UL CCs, this is called asymmetricaggregation. The term CA is interchangeable with carrier combining,bandwidth aggregation, spectrum aggregation, etc.

The LTE-A system aims to support a bandwidth of up to 100 MHz byaggregating two or more CCs, that is, by CA. To guarantee backwardcompatibility with a legacy IMT system, each of one or more carriers,which has a smaller bandwidth than a target bandwidth, may be limited toa bandwidth used in the legacy system.

For example, the legacy 3GPP LTE system supports bandwidths {1.4, 3, 5,10, 15, and 20 MHz} and the 3GPP LTE-A system may support a broaderbandwidth than 20 MHz using these LTE bandwidths. A CA system of thepresent disclosure may support CA by defining a new bandwidthirrespective of the bandwidths used in the legacy system.

There are two types of CA, intra-band CA and inter-band CA. Intra-bandCA means that a plurality of DL CCs and/or UL CCs are successive oradjacent in frequency. In other words, the carrier frequencies of the DLCCs and/or UL CCs are positioned in the same band. On the other hand, anenvironment where CCs are far away from each other in frequency may becalled inter-band CA. In other words, the carrier frequencies of aplurality of DL CCs and/or UL CCs are positioned in different bands. Inthis case, a UE may use a plurality of Radio Frequency (RF) ends toconduct communication in a CA environment.

The LTE-A system adopts the concept of cell to manage radio resources.The above-described CA environment may be referred to as a multi-cellenvironment. A cell is defined as a pair of DL and UL CCs, although theUL resources are not mandatory. Accordingly, a cell may be configuredwith DL resources alone or DL and UL resources.

For example, if one serving cell is configured for a specific UE, the UEmay have one DL CC and one UL CC. If two or more serving cells areconfigured for the UE, the UE may have as many DL CCs as the number ofthe serving cells and as many UL CCs as or fewer UL CCs than the numberof the serving cells, or vice versa. That is, if a plurality of servingcells are configured for the UE, a CA environment using more UL CCs thanDL CCs may also be supported.

CA may be regarded as aggregation of two or more cells having differentcarrier frequencies (center frequencies). Herein, the term ‘cell’ shouldbe distinguished from ‘cell’ as a geographical area covered by an eNB.Hereinafter, intra-band CA is referred to as intra-band multi-cell andinter-band CA is referred to as inter-band multi-cell.

In the LTE-A system, a Primacy Cell (PCell) and a Secondary Cell (SCell)are defined. A PCell and an SCell may be used as serving cells. For a UEin RRC_CONNECTED state, if CA is not configured for the UE or the UEdoes not support CA, a single serving cell including only a PCell existsfor the UE. On the contrary, if the UE is in RRC_CONNECTED state and CAis configured for the UE, one or more serving cells may exist for theUE, including a PCell and one or more SCells.

Serving cells (PCell and SCell) may be configured by an RRC parameter. Aphysical-layer ID of a cell, PhysCellId is an integer value ranging from0 to 503. A short ID of an SCell, SCellIndex is an integer value rangingfrom 1 to 7. A short ID of a serving cell (PCell or SCell),ServeCellIndex is an integer value ranging from 1 to 7. IfServeCellIndex is 0, this indicates a PCell and the values ofServeCellIndex for SCells are pre-assigned. That is, the smallest cellID (or cell index) of ServeCellIndex indicates a PCell.

A PCell refers to a cell operating in a primary frequency (or a primaryCC). A UE may use a PCell for initial connection establishment orconnection reestablishment. The PCell may be a cell indicated duringhandover. In addition, the PCell is a cell responsible forcontrol-related communication among serving cells configured in a CAenvironment. That is, PUCCH allocation and transmission for the UE maytake place only in the PCell. In addition, the UE may use only the PCellin acquiring system information or changing a monitoring procedure. AnEvolved Universal Terrestrial Radio Access Network (E-UTRAN) may changeonly a PCell for a handover procedure by a higher layerRRCConnectionReconfiguraiton message including mobilityControlInfo to aUE supporting CA.

An SCell may refer to a cell operating in a secondary frequency (or asecondary CC). Although only one PCell is allocated to a specific UE,one or more SCells may be allocated to the UE. An SCell may beconfigured after RRC connection establishment and may be used to provideadditional radio resources. There is no PUCCH in cells other than aPCell, that is, in SCells among serving cells configured in the CAenvironment.

When the E-UTRAN adds an SCell to a UE supporting CA, the E-UTRAN maytransmit all system information related to operations of related cellsin RRC_CONNECTED state to the UE by dedicated signaling. Changing systeminformation may be controlled by releasing and adding a related SCell.Herein, a higher layer RRCConnectionReconfiguration message may be used.The E-UTRAN may transmit a dedicated signal having a different parameterfor each cell rather than it broadcasts in a related SCell.

After an initial security activation procedure starts, the E-UTRAN mayconfigure a network including one or more SCells by adding the SCells toa PCell initially configured during a connection establishmentprocedure. In the CA environment, each of a PCell and an SCell mayoperate as a CC. Hereinbelow, a Primary CC (PCC) and a PCell may be usedin the same meaning and a Secondary CC (SCC) and an SCell may be used inthe same meaning in embodiments of the present disclosure.

FIG. 13 illustrates an example of CCs and CA in the LTE-A system, whichare used in embodiments of the present disclosure.

FIG. 13(a) illustrates a single carrier structure in the LTE system.There are a DL CC and a UL CC and one CC may have a frequency range of20 MHz.

FIG. 13(b) illustrates a CA structure in the LTE-A system. In theillustrated case of FIG. 13(b), three CCs each having 20 MHz areaggregated. While three DL CCs and three UL CCs are configured, thenumbers of DL CCs and UL CCs are not limited. In CA, a UE may monitorthree CCs simultaneously, receive a DL signal/DL data in the three CCs,and transmit a UL signal/UL data in the three CCs.

If a specific cell manages N DL CCs, the network may allocate M (M≤N) DLCCs to a UE. The UE may monitor only the M DL CCs and receive a DLsignal in the M DL CCs. The network may prioritize L (L≤M≤N) DL CCs andallocate a main DL CC to the UE. In this case, the UE should monitor theL DL CCs. The same thing may apply to UL transmission.

The linkage between the carrier frequencies of DL resources (or DL CCs)and the carrier frequencies of UL resources (or UL CCs) may be indicatedby a higher layer message such as an RRC message or by systeminformation. For example, a set of DL resources and UL resources may beconfigured based on linkage indicated by System Information Block Type 2(SIB2). Specifically, DL-UL linkage may refer to a mapping relationshipbetween a DL CC carrying a PDCCH with a UL grant and a UL CC using theUL grant, or a mapping relationship between a DL CC (or a UL CC)carrying HARQ data and a UL CC (or a DL CC) carrying an HARQ ACK/NACKsignal.

2.2 Cross Carrier Scheduling

Two scheduling schemes, self-scheduling and cross carrier scheduling aredefined for a CA system, from the perspective of carriers or servingcells. Cross carrier scheduling may be called cross CC scheduling orcross cell scheduling.

In self-scheduling, a PDCCH (carrying a DL grant) and a PDSCH aretransmitted in the same DL CC or a PUSCH is transmitted in a UL CClinked to a DL CC in which a PDCCH (carrying a UL grant) is received.

In cross carrier scheduling, a PDCCH (carrying a DL grant) and a PDSCHare transmitted in different DL CCs or a PUSCH is transmitted in a UL CCother than a UL CC linked to a DL CC in which a PDCCH (carrying a ULgrant) is received.

Cross carrier scheduling may be activated or deactivated UE-specificallyand indicated to each UE semi-statically by higher layer signaling (e.g.RRC signaling).

If cross carrier scheduling is activated, a Carrier Indicator Field(CIF) is required in a PDCCH to indicate a DL/UL CC in which aPDSCH/PUSCH indicated by the PDCCH is to be transmitted. For example,the PDCCH may allocate PDSCH resources or PUSCH resources to one of aplurality of CCs by the CIF. That is, when a PDCCH of a DL CC allocatesPDSCH or PUSCH resources to one of aggregated DL/UL CCs, a CIF is set inthe PDCCH. In this case, the DCI formats of LTE Release-8 may beextended according to the CIF. The CIF may be fixed to three bits andthe position of the CIF may be fixed irrespective of a DCI format size.In addition, the LTE Release-8 PDCCH structure (the same coding andresource mapping based on the same CCEs) may be reused.

On the other hand, if a PDCCH transmitted in a DL CC allocates PDSCHresources of the same DL CC or allocates PUSCH resources in a single ULCC linked to the DL CC, a CIF is not set in the PDCCH. In this case, theLTE Release-8 PDCCH structure (the same coding and resource mappingbased on the same CCEs) may be used.

If cross carrier scheduling is available, a UE needs to monitor aplurality of PDCCHs for DCI in the control region of a monitoring CCaccording to the transmission mode and/or bandwidth of each CC.Accordingly, an appropriate SS configuration and PDCCH monitoring areneeded for the purpose.

In the CA system, a UE DL CC set is a set of DL CCs scheduled for a UEto receive a PDSCH, and a UE UL CC set is a set of UL CCs scheduled fora UE to transmit a PUSCH. A PDCCH monitoring set is a set of one or moreDL CCs in which a PDCCH is monitored. The PDCCH monitoring set may beidentical to the UE DL CC set or may be a subset of the UE DL CC set.The PDCCH monitoring set may include at least one of the DL CCs of theUE DL CC set. Or the PDCCH monitoring set may be defined irrespective ofthe UE DL CC set. DL CCs included in the PDCCH monitoring set may beconfigured to always enable self-scheduling for UL CCs linked to the DLCCs. The UE DL CC set, the UE UL CC set, and the PDCCH monitoring setmay be configured UE-specifically, UE group-specifically, orcell-specifically.

If cross carrier scheduling is deactivated, this implies that the PDCCHmonitoring set is always identical to the UE DL CC set. In this case,there is no need for signaling the PDCCH monitoring set. However, ifcross carrier scheduling is activated, the PDCCH monitoring set may bedefined within the UE DL CC set. That is, the eNB transmits a PDCCH onlyin the PDCCH monitoring set to schedule a PDSCH or PUSCH for the UE.

FIG. 14 illustrates a cross carrier-scheduled subframe structure in theLTE-A system, which is used in embodiments of the present disclosure.

Referring to FIG. 14, three DL CCs are aggregated for a DL subframe forLTE-A UEs. DL CC ‘A’ is configured as a PDCCH monitoring DL CC. If a CIFis not used, each DL CC may deliver a PDCCH that schedules a PDSCH inthe same DL CC without a CIF. On the other hand, if the CIF is used byhigher layer signaling, only DL CC ‘A’ may carry a PDCCH that schedulesa PDSCH in the same DL CC ‘A’ or another CC. Herein, no PDCCH istransmitted in DL CC ‘B’ and DL CC ‘C’ that are not configured as PDCCHmonitoring DL CCs.

FIG. 15 is conceptual diagram illustrating a construction of servingcells according to cross-carrier scheduling.

Referring to FIG. 15, an eNB (or BS) and/or UEs for use in a radioaccess system supporting carrier aggregation (CA) may include one ormore serving cells. In FIG. 8, the eNB can support a total of fourserving cells (cells A, B, C and D). It is assumed that UE A may includeCells (A, B, C), UE B may include Cells (B, C, D), and UE C may includeCell B. In this case, at least one of cells of each UE may be composedof P Cell. In this case, P Cell is always activated, and SCell may beactivated or deactivated by the eNB and/or UE.

The cells shown in FIG. 15 may be configured per UE. The above-mentionedcells selected from among cells of the eNB, cell addition may be appliedto carrier aggregation (CA) on the basis of a measurement report messagereceived from the UE. The configured cell may reserve resources forACK/NACK message transmission in association with PDSCH signaltransmission. The activated cell is configured to actually transmit aPDSCH signal and/or a PUSCH signal from among the configured cells, andis configured to transmit CSI reporting and Sounding Reference Signal(SRS) transmission. The deactivated cell is configured not totransmit/receive PDSCH/PUSCH signals by an eNB command or a timeroperation, and CRS reporting and SRS transmission are interrupted.

2.4. Channel State Information (CSI) Feedback on PUCCH

First of all, in the 3GPP LTE system, when a DL reception entity (e.g.,UE) is connected to a DL transmission entity (e.g., BS), the DLreception entity performs measurement on a Reference Signal ReceivedPower (RSRP) of a reference signal transmitted in DL, a quality of areference signal (RSRQ: Reference Signal Received Quality) and the likeat a random time and is then able to make a periodic or even-triggeredreport of a corresponding measurement result to the BS.

Each UE reports a DL channel information in accordance with a DL channelstatus via uplink. A base station is then able to determinetime/frequency resources, MCS (modulation and coding scheme) and thelike appropriate for a data transmission to each UE using the DL channelinformation received from the each UE.

Such Channel State Information (CSI) may include Channel QualityIndicator (CQI), Precoding Matrix Indicator (PMI), Precoder TypeIndication (PTI) and/or Rank Indication (RI). In particular, the CSI maybe transmitted entirely or partially depending on a transmission mode ofeach UE. CQI is determined based on a received signal quality of a UE,which may be generally determined on the basis of a measurement of a DLreference signal. In doing so, a CQI value actually delivered to a basestation may correspond to an MCS capable of providing maximumperformance by maintaining a Block Error Rate (BLER) under 10% in thereceived signal quality measured by a UE.

This channel information reporting may be classified into a periodicreport transmitted periodically and an aperiodic report transmitted inresponse to a request made by a BS.

In case of the aperiodic report, it is set for each UE by a 1-bitrequest bit (CQI request bit) contained in UL scheduling informationdownloaded to a UE by a BS. Having received this information, each UE isthen able to deliver channel information to the BS via a Physical UplinkShared Channel (PUSCH) in consideration of its transmission mode. And,it may set RI and CQI/PMI not to be transmitted on the same PUSCH.

In case of the periodic report, a period for transmitting channelinformation via an upper layer signal, an offset in the correspondingperiod and the like are signaled to each UE by subframe unit and channelinformation in consideration of a transmission mode of each UE may bedelivered to a BS via a Physical Uplink Control Channel (PUCCH) inaccordance with a determined period. In case that data transmitted inuplink simultaneously exists in a subframe in which channel informationis transmitted by a determined period, the corresponding channelinformation may be transmitted together with the data not on the PUCCHbut on a Physical Uplink Shared Channel (PUSCH). In case of the periodicreport via PUCCH, bits (e.g., 11 bits) limited further than those of thePUSCH may be used. RI and CQI/PMI may be transmitted on the same PUSCH.

In case that contention occurs between the periodic report and theaperiodic report in the same subframe, only the aperiodic report can beperformed.

In calculating Wideband CQI/PMI, a most recently transmitted RI may beusable. RI in a PUCCH CSI report mode is independent from RI in a PUSCHCSI report mode. The RI in the PUSCH CSI report mode is valid forCQI/PMI in the corresponding PUSCH CSI report mode only.

Table 16 is provided to describe CSI feedback type transmitted on PUCCHand PUCCH CSI report mode.

TABLE 16 PMI Feedback Type No PMI (OL, TD, single-antenna) Single PMI(CL) CQI Wideband Mode 1-0 Mode 1-1 Feedback RI (only for Open-Loop SM)RI Type One Wideband CQI (4 bit) Wideband CQI (4 bit) when RI > 1, CQIof first codeword Wideband spatial CQI (3 bit) for RI > 1 Wideband PMI(4 bit) UE Mode 2-0 Mode 2-1 Selected RI (only for Open-Loop SM) RIWideband CQI (4 bit) Wideband CQI (4 bit) Best-1 CQI (4 bit) in each BPWideband spatial CQI (3 bit) for RI > 1 Best-1 indicator(L-bit label)Wideband PMI (4 bit) when RI > 1, CQI of first codeword Best-1 CQI (4bit) 1 in each BP Best-1 spatial CQI (3 bit) for RI > 1 Best-1 indicator(L-bit label)

Referring to [Table 16], in the periodic report of channel information,there are 4 kinds of reporting modes (mode 1-0, mode 1-2, mode 2-0 andmode 2-1) in accordance with CQI and PMI feedback types.

CQI can be classified into WideBand (WB) CQI and SubBand (SB) CQI inaccordance with CQI feedback type and PMI can be classified into No PMIor Single PMI in accordance with a presence or non-presence of PMItransmission. In Table 11, No PMI corresponds to a case of Open-Loop(OL), Transmit Diversity (TD) and single-antenna, while Single PMIcorresponds to a case of Closed-Loop (CL).

The mode 1-0 corresponds to a case that WB CQI is transmitted in theabsence of PMI transmission. In this case, RI is transmitted only incase of OL Spatial Multiplexing (SM) and one WB CQI represented as 4bits can be transmitted. If RI is greater than 1, CQI for a 1^(st)codeword can be transmitted.

Mode 1-1 corresponds to a case that a single PMI and WB CQI aretransmitted. In this case, 4-bit WB CQI and 4-bit WB PMI can betransmitted together with RI transmission. Additionally, if RI isgreater than 1, 3-bit WB (wideband) spatial differential CQI can betransmitted. In 2-codeword transmission, the WB spatial differential CQImay indicate a difference value between a WB CQI index for codeword 1and a WB CQI index for codeword 2. The difference value in-between mayhave a value selected from a set {−4, −3, −2, −1, 0, 1, 2, 3} and can berepresented as 3 bits.

The mode 2-0 corresponds to a case that CQI on a UE-selected band istransmitted in the absence of PMI transmission. In this case, RI istransmitted only in case of open-loop spatial multiplexing (SM) and a WBCQI represented as 4 bits may be transmitted. A best CQI (best-1) istransmitted on each bandwidth part (BP) and the best-1 CQI may berepresented as 4 bits. And, an L-bit indicator indicating the best-1 maybe transmitted together. If the RI is greater than 1, a CQI for a 1^(st)codeword can be transmitted.

And, Mode 2-1 corresponds to a case that a single PMI and a CQI on aUE-selected band are transmitted. In this case, together with RItransmission, 4-bit WB CQI, 3-bit WB spiral differential CQI and 4-bitWB PMI can be transmitted. Additionally, 4-bit best-1 CQI is transmittedon each Bandwidth Part (BP) and L-bit best-1 indicator can betransmitted together. Additionally, if RI is greater than 1, 3-bitbest-1 spatial differential CQI can be transmitted. In 2-codewordtransmission, it may indicate a difference value between a best-1 CQIindex of codeword 1 and a best-1 CQI index of codeword 2.

For the transmission modes, periodic PUCCH CSI report modes aresupported as follows.

1) Transmission mode 1: Modes 1-0 and 2-0

2) Transmission mode 2: Modes 1-0 and 2-0

3) Transmission mode 3: Modes 1-0 and 2-0

4) Transmission mode 4: Modes 1-1 and 2-1

5) Transmission mode 5: Modes 1-1 and 2-1

6) Transmission mode 6: Modes 1-1 and 2-1

7) Transmission mode 7: Modes 1-0 and 2-0

8) Transmission mode 8: Modes 1-1 and 2-1 if a UE is set to make aPMI/RI reporting, or Modes 1-0 and 2-0 if a UE is set not to make aPMI/RI reporting

9) Transmission mode 9: Modes 1-1 and 2-1 if a UE is set to make aPMI/RI reporting and the number of CSI-RS ports is greater than 1, orModes 1-0 and 2-0 if a UE is set not to make a PMI/RI reporting and thenumber of CSI-RS port(s) is equal to 1.

The periodic PUCCH CSIU reporting mode in each serving cell is set byupper layer signaling. And, Mode 1-1 is set to either submode 1 orsubmode 2 by an upper layer signaling using a parameter‘PUCCH_format1-1_CSI_reporting_mode’.

A CQI reporting in a specific subframe of a specific serving cell in aUE-selected SB CQI means a measurement of at least one channel state ofa bandwidth part (BP) corresponding to a portion of a bandwidth of aserving cell. An index is given to the bandwidth part in a frequencyincreasing order starting with a lowest frequency without an incrementof a bandwidth.

2.4 Method for Transmitting ACK/NACK on PUCCH

2.4.1 ACK/NACK Transmission in LTE System

Under the situation that a UE simultaneously transmits a plurality ofACKs/NACKs corresponding to multiple data units received from an eNB, inorder to maintain the single-carrier property of ACK/NACK signals andreduce the total ACK/NACK transmission power, ACK/NACK multiplexingmethod based on PUCCH resource selection can be considered. WithACK/NACK multiplexing, contents of the ACK/NACK signals for multipledata units are identified by the combination of the PUCCH resource usedin actual ACK/NACK transmission and the one of QPSK modulation symbols.For example, if it is assumed that one PUCCH resource carries 4 bits and4 data units can be transmitted in maximum (at this time, assume thatHARQ operation for each data unit can be managed by single ACK/NACKbit), the Transmission (Tx) node can identify the ACK/NACK result basedon the transmission position of the PUCCH signal and the bits of theACK/NACK signal as shown in [Table 17] below.

TABLE 17 HARQ-ACK(0), HARQ-ACK(1), b(0), HARQ-ACK(2), HARQ-ACK(3)n_(PUCCH) ⁽¹⁾ b(1) ACK, ACK, ACK, ACK n_(PUCCH, 1) ⁽¹⁾ 1, 1 ACK, ACK,ACK, NACK/DTX n_(PUCCH, 1) ⁽¹⁾ 1, 0 NACK/DTX, NACK/DTX, NACK, DTXn_(PUCCH, 2) ⁽¹⁾ 1, 1 ACK, ACK, NACK/DTX, ACK n_(PUCCH, 1) ⁽¹⁾ 1, 0NACK, DTX, DTX, DTX n_(PUCCH, 0) ⁽¹⁾ 1, 0 ACK, ACK, NACK/DTX, NACK/DTXn_(PUCCH, 1) ⁽¹⁾ 1, 0 ACK, NACK/DTX, ACK, ACK n_(PUCCH, 3) ⁽¹⁾ 0, 1NACK/DTX, NACK/DTX, NACK/DTX, NACK n_(PUCCH, 3) ⁽¹⁾ 1, 1 ACK, NACK/DTX,ACK, NACK/DTX n_(PUCCH, 2) ⁽¹⁾ 0, 1 ACK, NACK/DTX, NACK/DTX, ACKn_(PUCCH, 0) ⁽¹⁾ 0, 1 ACK, NACK/DTX, NACK/DTX, NACK/DTX n_(PUCCH, 0) ⁽¹⁾1, 1 NACK/DTX, ACK, ACK, ACK n_(PUCCH, 3) ⁽¹⁾ 0, 1 NACK/DTX, NACK, DTX,DTX n_(PUCCH, 1) ⁽¹⁾ 0, 0 NACK/DTX, ACK, ACK, NACK/DTX n_(PUCCH, 2) ⁽¹⁾1, 0 NACK/DTX, ACK, NACK/DTX, ACK n_(PUCCH, 3) ⁽¹⁾ 1, 0 NACK/DTX, ACK,NACK/DTX, NACK/DTX n_(PUCCH, 1) ⁽¹⁾ 0, 1 NACK/DTX, NACK/DTX, ACK, ACKn_(PUCCH, 3) ⁽¹⁾ 0, 1 NACK/DTX, NACK/DTX, ACK, NACK/DTX n_(PUCCH, 2) ⁽¹⁾0, 0 NACK/DTX, NACK/DTX, NACK/DTX, ACK n_(PUCCH, 3) ⁽¹⁾ 0, 0 DTX, DTX,DTX, DTX N/A N/A

In [Table 17], HARQ-ACK(i) indicates the ACK/NACK result for the dataunit i. For example, if a maximum of 4 data units is transmitted, i=0,1, 2, 3. In Table 17, DTX means that there is no data unit transmittedfor corresponding HARQ-ACK(i) or the Reception (Rx) node doesn't detectthe existence of the data unit corresponding to HARQ-ACK(i).

In addition, n_(PUCCH,X) ⁽¹⁾ indicates the PUCCH resource which shouldbe used in actual ACK/NACK transmission, if there are 4 PUCCH resources,a maximum of four PUCCH resources n_(PUCCH,0) ⁽¹⁾, n_(PUCCH,1) ⁽¹⁾,n_(PUCCH,2) ⁽¹⁾, and n_(PUCCH,3) ⁽¹⁾ may be allocated to the UE.

In addition, b(0), b(1) indicates two bits carried by the selected PUCCHresource. Modulation symbols which are transmitted through PUCCHresource are decided according to the bits. For example, if the RX nodereceives and decodes 4 data units successfully, the RX node shouldtransmit two bits, (1, 1), using PUCCH resource n_(PUCCH,1) ⁽¹⁾. Foranother example, if the RX node receives 4 data units and fails indecoding the first and the third data units (corresponding toHARQ-ACK(0) and HARQ-ACK(2)), the RX node should transmit (1, 0) usingn_(PUCCH,3) ⁽¹⁾.

By linking the actual ACK/NACK contents with the combination of PUCCHresource selection and the actual bit contents in the transmitted PUCCHresource in this way, ACK/NACK transmission using a single PUCCHresource for multiple data units is possible.

In ACK/NACK multiplexing method (see Table 17), basically, NACK and DTXare coupled as NACK/DTX if at least one ACK exists for all data units.This is because combinations of PUCCH resources and QPSK symbols areinsufficient to cover all ACK, NACK and DTX hypotheses. On the otherhand, for the case that no ACK exists for all data units (in otherwords, NACK or DTX only exists for all data units), single NACKdecoupled with DTX is defined one as HARQ-ACK(i). In this case, PUCCHresource linked to the data unit corresponding to single NACK can bealso reserved to transmit the signal of multiple ACKs/NACKs.

2.4.2 ACK/NACK Transmission in LTE-A System

In an LTE-A system (e.g., Rel-10, 11, 12, etc.), transmission of aplurality of ACK/NACK signals for a plurality of PDSCH signals, which istransmitted via a plurality of DL CCs, via a specific UL CC isconsidered. Unlike ACK/NACK transmission using PUCCH format 1a/1b of anLTE system, a plurality of ACK/NACK signals may be subjected to channelcoding (e.g., Reed-Muller coding, Tail-biting convolutional coding,etc.) and then a plurality of ACK/NACK information/signals may betransmitted using PUCCH format 2 or a new PUCCH format (e.g., an E-PUCCHformat) modified based on block spreading.

FIG. 16 shows an example of a new PUCCH format based on block spreading.

A block spreading scheme refers to a method for performing modulationusing an SC-FDMA scheme unlike PUCCH format series 1 or 2 in an LTEsystem. The block spreading scheme refers to a scheme for time-domainspreading and transmitting a symbol sequence using an Orthogonal CoverCode (OCC) as shown in FIG. 16. That is, the symbol sequence is spreadusing the OCC to multiplex control signals of several UEs in the sameRB.

In the above-described PUCCH format 2, one symbol sequence istransmitted over the time domain and UE multiplexing is performed usingCyclic Shift (CCS) of a CAZAC sequence. However, in the new PUCCH formatbased on block spreading, one symbol sequence is transmitted over thefrequency domain and UE multiplexing is performed using time-domainspreading based on an OCC.

For example, as shown in FIG. 16, one symbol sequence may be generatedas five SC-FDMA symbols by an OCC of length-5 (that is, SF=5). Althougha total of 2 RS symbols is used during one slot in FIG. 16, variousmethods using three RS symbols and using an OCC of SF=4 may be used. Atthis time, the RS symbols may be generated from a CAZAC sequence havingspecific cyclic shift and may be transmitted in the form in which aspecific OCC is applied (multiplied by) to a plurality of RS symbols ofthe time domain.

In the embodiments of the present invention, for convenience ofdescription, a multi-ACK/NACK transmission scheme based on channelcoding using PUCCH format 2 or a new PUCCH format (e.g., an E-PUCCHformat) is defined as a “multi-bit ACK/NACK coding transmission method”.

The multi-bit ACK/NACK coding method refers to a method for transmittingACK/NACK code blocks generated by channel-coding ACK/NACK or DTXinformation (meaning that the PDCCH is not received/detected) for PDSCHsignals transmitted on a plurality of DL CCs.

For example, when the UE operates on a certain DL CC in an SU-MIMO modeand receives two CodeWords (CW), the UE may have a maximum of fivefeedback states including a total of four feedback states of each CW,such as ACK/ACK, ACK/NACK, NACK/ACK and NACK/NACK, and DTX. When the UEreceives a single CW, the UE may have a maximum of three statesincluding ACK, NACK and/or DTX. When NACK and DTX are equally processed,the UE may have a total of two states such as ACK and NACK/DTX.

Accordingly, when the UE aggregates a maximum of five DL CCs and the UEoperates on all DL CCs in an SU-MIMO mode, the UE may have a maximum of55 transmittable feedback states. At this time, the size of ACK/NACKpayload representing the 55 feedback states may be a total of 12 bits.If DTX and NACK are equally processed, the number of feedback statesbecomes 45 and the size of the ACK/NACK payload representing thefeedback states is a total of 10 bits.

In an ACK/NACK multiplexing (that is, ACK/NACK selection) method appliedto an LTE TDD system, fundamentally, an implicit ACK/NACK selectionmethod in which an implicit PUCCH resource corresponding to a PDCCHscheduling each PDSCH (that is, linked to a smallest CCE index) is usedfor ACK/NACK transmission in order to secure a PUCCH resource of eachUE.

In an LTE-A FDD system, transmission of a plurality of ACK/NACK signalsfor a plurality of PDSCH signals transmitted via a plurality of DL CCsvia one UE-specific UL CC is considered. “ACK/NACK selection” methodsusing an implicit PUCCH resource linked to a PDCCH scheduling some orall DL CCs (that is, linked to a smallest CCE index nCCE or linked tonCCE and nCCE+1) or a combination of an implicit PUCCH and an explicitPUCCH resource pre-allocated to each UE via RRC signaling areconsidered.

Even in an LTE-A TDD system, aggregation of a plurality of CCs isconsidered. For example, when a plurality of CCs is aggregated, UEtransmitting a plurality of ACK/NACK information/signals for a pluralityof PDSCH signals transmitted via a plurality of DL subframes and aplurality of CCs via a specific CC (that is, A/N CC) in UL subframescorresponding to the plurality of DL subframes in which the PDSCHsignals are transmitted is considered.

At this time, unlike LTE-A FDD, a method (that is, full ACK/NACK) fortransmitting a plurality of ACK/NACK signals corresponding to a maximumnumber of CWs, which may be transmitted via all CCs allocated to the UE,for a plurality of DL subframes may be considered or a method (that is,bundled ACK/NACK) for applying ACK/NACK bundling to a CW, CC and/or asubframe region, reducing the number of transmitted ACKs/NACKs andperforming transmission may be considered.

At this time, CW bundling means that ACK/NACK bundling for CW per CC isapplied to each DL subframe and CC bundling means that ACK/NACK bundlingfor all or some CCs is applied to each DL subframe. In addition,subframe bundling means that ACK/NACK bundling for all or some DLsubframes is applied to each CC.

As the subframe bundling method, an ACK counter method indicating atotal number of ACKs (or the number of some ACKs) per CC for all PDSCHsignals or DL grant PDCCHs received on each DL CC may be considered. Atthis time, the multi-bit ACK/NACK coding scheme or the ACK/NACKtransmission scheme based on the ACK/NACK selection method may beconfigurably applied according to the size of the ACK/NACK payload perUE, that is, the size of the ACK/NACK payload for transmission of fullor bundled ACK/NACK configured per UE.

2.5 Procedure for Transmitting and Receiving PUCCH

In a mobile communication system, one eNB transmits and receives data toand from a plurality of UEs via a wireless channel environment in onecell/sector. In a system operating using multiple carriers or the like,the eNB receives packet traffic from a wired Internet network andtransmits the received packet traffic to each UE using a predeterminedcommunication scheme. At this time, downlink scheduling is how the eNBdetermines when data is transmitted to which UE using which frequencydomain. In addition, the eNB receives and demodulates data from the UEusing a predetermined communication scheme and transmits packet trafficover a wired Internet network. Uplink scheduling is how the eNBdetermines when to enable which UE to transmit uplink data using whichfrequency domain. In general, a UE having a good channel state maytransmit and receive data using more time and frequency resources.

In a system operating using multiple carriers or the like, resources maybe roughly divided into a time domain and a frequency domain. Theresources may be defined as resource blocks, which includes Nsubcarriers and M subframes or predetermined time units. At this time, Nand M may be 1. FIG. 17 is a diagram showing an example of configuring aresource block in time-frequency units.

In FIG. 17, one rectangle means one resource block and one resourceblock has several subcarriers on one axis and has a predetermined timeunit (e.g., slots or subframes) on the other axis.

In downlink, an eNB schedules one or more resource blocks to a UEselected according to a determined scheduling rule and transmits datausing resource bocks allocated to the UE. In uplink, the eNB schedulesone or more resource blocks to a UE selected according to apredetermined scheduling rule and a UE transmits data in uplink usingthe allocated resources.

An error control method performed when a (sub)frame, in which data istransmitted and received, is lost or damaged after transmitting andreceiving data after scheduling includes an Automatic Repeat reQuest(ARQ) method and a Hybrid ARQ (HARQ) method.

In the ARQ method, fundamentally, a transmitter waits for anacknowledgement (ACK) message after transmitting one (sub)frame and areceiver sends the ACK only upon receiving the sub(frame). When an erroroccurs in the (sub)frame, a negative ACK (NAK) message is sent andinformation on a reception frame, in which an error occurs, is removedfrom a receiver buffer. The transmitter transmits a subsequent(sub)frame upon receiving the ACK message but retransmits the (sub)frameupon receiving the NAK message. Unlike the ARQ method, in the HARQmethod, when the received frame cannot be demodulated, the receivertransmits the NAK message to the transmitter, but the received frame isstored in a buffer during a predetermined time and is combined with aretransmitted frame, thereby increasing a reception success rate.

Recently, a HARQ method more efficient than the ARQ method is widelyused. The HARQ method may be divided into various methods. For example,the HARQ method may be divided into a synchronous HARQ method and anasynchronous HARQ method according to retransmission timing and into achannel-adaptive HARQ method and a channel-non-adaptive HARQ methoddepending on whether the amount of resources used for retransmission isinfluenced by a channel state.

The synchronous HARQ method refers to a method of performing subsequentretransmission at timing determined by a system when initialtransmission fails. For example, if it is assumed that retransmission isperformed every four time units after initial transmission fails,retransmission timing is predetermined between the eNB and the UE and isnot signaled. However, when the data transmission side receives a NAKmessage, the frame is retransmitted every four time units until an ACKmessage is received.

Meanwhile, the asynchronous HARQ method may be performed by newlyscheduling retransmission timing or via additional signaling. Theretransmission timing of the previously failed frame may be changed byseveral factors such as channel state.

The channel-non-adaptive HARQ method refers to a method of usingscheduling information (e.g., the modulation method of the frame, thenumber of used resource blocks, Adaptive Modulation and Coding (AMC),etc.), which is set upon initial transmission, upon retransmission. Incontrast, the channel-adaptive HARQ method refers to a method ofchanging such scheduling information according to the channel state.

For example, in the channel-non-adaptive HARQ method, a transmissionside transmits data using six resource blocks upon initial transmissionand retransmits data using six resource blocks upon retransmission. Incontrast, in the channel-adaptive HARQ method, initial transmission isperformed using six resource blocks and retransmission is performedusing greater or less than six resource blocks according to the channelstate.

Although there are four HARQ methods, the asynchronous andchannel-adaptive HARQ method and the synchronous andchannel-non-adaptive HARQ method are mainly used. The asynchronous andchannel-adaptive HARQ method may maximize retransmission efficiency byadaptively changing the retransmission timing and the amount of usedresources according to the channel state but may increase overhead.Accordingly, the asynchronous and channel-adaptive HARQ method is notgenerally considered for uplink. In contrast, the synchronous andchannel-non-adaptive HARQ method may not cause overhead becauseretransmission timing and resource allocation are predetermined in thesystem, but has very low retransmission efficiency in a considerablychanged channel state.

To this end, in the current 3GPP LTE/LTE-A system, the asynchronous HARQmethod is used in downlink and the synchronous HARQ method is used inuplink.

FIG. 18 is a diagram showing an example of a resource allocation andretransmission method of an asynchronous HARQ method.

When an eNB transmits scheduling information in downlink, receivesACK/NAK information from a UE, and transmits next data, time delayoccurs as shown in FIG. 19. This is channel propagation delay and delayoccurring due to a time required for data decoding and data encoding.

A method of performing transmission using an independent HARQ processfor data transmission without a gap during a delay period is being used.For example, if a shortest period from first data transmission to nextdata transmission is 7 subframes, data may be transmitted without a gapby setting 7 independent HARQ processes. In an LTE/LTE-A system, amaximum of eight HARQ processes may be allocated to one UE in non-MIMO.

2.6 CA Environment-Based CoMP Operation

Hereinafter, a cooperation multi-point (CoMP) transmission operationapplicable to the embodiments of the present disclosure will bedescribed.

In the LTE-A system, CoMP transmission may be implemented using acarrier aggregation (CA) function in the LTE. FIG. 19 is a conceptualview illustrating a CoMP system operating based on a CA environment.

In FIG. 19, it is assumed that a carrier operated as a PCell and acarrier operated as an SCell may use the same frequency band on afrequency axis and are allocated to two eNBs geographically spaced apartfrom each other. At this time, a serving eNB of UE1 may be allocated tothe PCell, and a neighboring cell causing much interference may beallocated to the SCell. That is, the eNB of the PCell and the eNB of theSCell may perform various DL/UL CoMP operations such as jointtransmission (JT), CS/CB and dynamic cell selection for one UE.

FIG. 19 illustrates an example that cells managed by two eNBs areaggregated as PCell and SCell with respect to one UE (e.g., UE1).However, as another example, three or more cells may be aggregated. Forexample, some cells of three or more cells may be configured to performCoMP operation for one UE in the same frequency band, and the othercells may be configured to perform simple CA operation in differentfrequency bands. At this time, the PCell does not always need toparticipate in CoMP operation.

2.7 Reference Signal (RS)

Now, a description will be given of RSs which may be used in embodimentsof the present disclosure.

FIG. 20 illustrates an example of a subframe to which UE-RSs areallocated, which may be used in embodiments of the present disclosure.

Referring to FIG. 20, the subframe illustrates REs occupied by UE-RSsamong REs in one RB of a normal DL subframe having a normal CP.

UE-RSs are transmitted on antenna port(s) p=5, p=7, p=8 or p=7, 8, . . ., υ+6 for PDSCH transmission, where i) is the number of layers used forthe PDSCH transmission. UE-RSs are present and are a valid reference forPDSCH demodulation only if the PDSCH transmission is associated with thecorresponding antenna port. UE-RSs are transmitted only on RBs to whichthe corresponding PDSCH is mapped.

The UE-RSs are configured to be transmitted only on RB(s) to which aPDSCH is mapped in a subframe in which the PDSCH is scheduled unlikeCRSs configured to be transmitted in every subframe irrespective ofwhether the PDSCH is present. Accordingly, overhead of the RS maydecrease relative to overhead of the CRS.

In the 3GPP LTE-A system, the UE-RSs are defined in a PRB pair.Referring to FIG. 19, in a PRB having frequency-domain index nPRBassigned for PDSCH transmission with respect to p=7, p=8, or p=7, 8, . .. , υ+6, a part of UE-RS sequence r(m) is mapped to complex-valuedmodulation symbols.

UE-RSs are transmitted through antenna port(s) correspondingrespectively to layer(s) of a PDSCH. That is, the number of UE-RS portsis proportional to a transmission rank of the PDSCH. Meanwhile, if thenumber of layers is 1 or 2, 12 REs per RB pair are used for UE-RStransmission and, if the number of layers is greater than 2, 24 REs perRB pair are used for UE-RS transmission. In addition, locations of REsoccupied by UE-RSs (i.e. locations of UE-RS REs) in a RB pair are thesame with respect to a UE-RS port regardless of a UE or a cell.

As a result, the number of DM-RS REs in an RB to which a PDSCH for aspecific UE in a specific subframe is mapped is the same per UE-RSports. Notably, in RBs to which the PDSCH for different UEs in the samesubframe is allocated, the number of DM-RS REs included in the RBs maydiffer according to the number of transmitted layers.

The UE-RS can be used as the DM-RS in the embodiments of the presentdisclosure.

2.8 Enhanced PDCCH (EPDCCH)

In the 3GPP LTE/LTE-A system, Cross-Carrier Scheduling (CCS) in anaggregation status for a plurality of component carriers (CC: componentcarrier=(serving) cell) will be defined. One scheduled CC may previouslybe configured to be DL/UL scheduled from another one scheduling CC (thatis, to receive DL/UL grant PDCCH for a corresponding scheduled CC). Atthis time, the scheduling CC may basically perform DL/UL scheduling foritself. In other words, a Search Space (SS) for a PDCCH for schedulingscheduling/scheduled CCs which are in the CCS relation may exist in acontrol channel region of all the scheduling CCs.

Meanwhile, in the LTE system, FDD DL carrier or TDD DL subframes areconfigured to use first n (n<=4) OFDM symbols of each subframe fortransmission of physical channels for transmission of various kinds ofcontrol information, wherein examples of the physical channels include aPDCCH, a PHICH, and a PCFICH. At this time, the number of OFDM symbolsused for control channel transmission at each subframe may be deliveredto the UE dynamically through a physical channel such as PCFICH orsemi-statically through RRC signaling.

Meanwhile, in the LTE/LTE-A system, since a PDCCH which is a physicalchannel for DL/UL scheduling and transmitting various kinds of controlinformation has a limitation that it is transmitted through limited OFDMsymbols, enhanced PDCCH (i.e., E-PDCCH) multiplexed with a PDSCH morefreely in a way of FDM/TDM may be introduced instead of a controlchannel such as PDCCH, which is transmitted through OFDM symbol andseparated from PDSCH. FIG. 21 illustrates an example that legacy PDCCH,PDSCH and E-PDCCH, which are used in an LTE/LTE-A system, aremultiplexed.

3. LTE-U System

3.1 LTE-U System Configuration

Hereinafter, methods for transmitting and receiving data in a CAenvironment of an LTE-A band corresponding to a licensed band and anunlicensed band will be described. In the embodiments of the presentdisclosure, an LTE-U system means an LTE system that supports such a CAstatus of a licensed band and an unlicensed band. A WiFi band orBluetooth (BT) band may be used as the unlicensed band. LTE-A systemoperating in an unlicensed band is referred to as LAA (licensed assistedaccess). Or, the LAA may correspond to a scheme of performing datatransmission/reception in an unlicensed band in a manner of beingcombined with a licensed band.

FIG. 22 illustrates an example of a CA environment supported in an LTE-Usystem.

Hereinafter, for convenience of description, it is assumed that a UE isconfigured to perform wireless communication in each of a licensed bandand an unlicensed band by using two CCs. The methods which will bedescribed hereinafter may be applied to even a case where three or moreCCs are configured for a UE.

In the embodiments of the present disclosure, it is assumed that acarrier of the licensed band may be a primary CC (PCC or PCell), and acarrier of the unlicensed band may be a secondary CC (SCC or SCell).However, the embodiments of the present disclosure may be applied toeven a case where a plurality of licensed bands and a plurality ofunlicensed bands are used in a carrier aggregation method. Also, themethods suggested in the present disclosure may be applied to even a3GPP LTE system and another system.

In FIG. 22, one eNB supports both a licensed band and an unlicensedband. That is, the UE may transmit and receive control information anddata through the PCC which is a licensed band, and may also transmit andreceive control information and data through the SCC which is anunlicensed band. However, the status shown in FIG. 22 is only example,and the embodiments of the present disclosure may be applied to even aCA environment that one UE accesses a plurality of eNBs.

For example, the UE may configure a macro eNB (M-eNB) and a PCell, andmay configure a small eNB (S-eNB) and an SCell. At this time, the macroeNB and the small eNB may be connected with each other through abackhaul network.

In the embodiments of the present disclosure, the unlicensed band may beoperated in a contention-based random access method. At this time, theeNB that supports the unlicensed band may perform a Carrier Sensing (CS)procedure prior to data transmission and reception. The CS proceduredetermines whether a corresponding band is reserved by another entity.

For example, the eNB of the SCell checks whether a current channel isbusy or idle. If it is determined that the corresponding band is idlestate, the eNB may transmit a scheduling grant to the UE to allocate aresource through (E)PDCCH of the PCell in case of a cross carrierscheduling mode and through PDCCH of the SCell in case of aself-scheduling mode, and may try data transmission and reception.

At this time, the eNB may configure a TxOP including N consecutivesubframes. In this case, a value of N and a use of the N subframes maypreviously be notified from the eNB to the UE through higher layersignaling through the PCell or through a physical control channel orphysical data channel.

3.2 Carrier Sensing (CS) Procedure

In embodiments of the present disclosure, a CS procedure may be called aClear Channel Assessment (CCA) procedure. In the CCA procedure, it maybe determined whether a channel is busy or idle based on a predeterminedCCA threshold or a CCA threshold configured by higher-layer signaling.For example, if energy higher than the CCA threshold is detected in anunlicensed band, SCell, it may be determined that the channel is busy oridle. If the channel is determined to be idle, an eNB may start signaltransmission in the SCell. This procedure may be referred to as LBT.

FIG. 23 is a view illustrating an exemplary Frame Based Equipment (FBE)operation as one of LBT operations.

The European Telecommunication Standards Institute (ETSI) regulation (EN301 893 V1.7.1) defines two LBT operations, Frame Based Equipment (FBE)and Load Based Equipment (LBE). In FBE, one fixed frame is comprised ofa channel occupancy time (e.g., 1 to 10 ms) being a time period duringwhich a communication node succeeding in channel access may continuetransmission, and an idle period being at least 5% of the channeloccupancy time, and CCA is defined as an operation for monitoring achannel during a CCA slot (at least 20 μs) at the end of the idleperiod.

A communication node periodically performs CCA on a per-fixed framebasis. If the channel is unoccupied, the communication node transmitsdata during the channel occupancy time. On the contrary, if the channelis occupied, the communication node defers the transmission and waitsuntil the CCA slot of the next period.

FIG. 24 is a block diagram illustrating the FBE operation.

Referring to FIG. 24, a communication node (i.e., eNB) managing an SCellperforms CCA during a CCA slot. If the channel is idle, thecommunication node performs data transmission (Tx). If the channel isbusy, the communication node waits for a time period calculated bysubtracting the CCA slot from a fixed frame period, and then resumesCCA.

The communication node transmits data during the channel occupancy time.Upon completion of the data transmission, the communication node waitsfor a time period calculated by subtracting the CCA slot from the idleperiod, and then resumes CCA. If the channel is idle but thecommunication node has no transmission data, the communication nodewaits for the time period calculated by subtracting the CCA slot fromthe fixed frame period, and then resumes CCA.

FIG. 25 is a view illustrating an exemplary LBE operation as one of theLBT operations.

Referring to FIG. 25(a), in LBE, the communication node first sets q (qϵ{4, 5, . . . , 32}) and then performs CCA during one CCA slot.

FIG. 25(b) is a block diagram illustrating the LBE operation. The LBEoperation will be described with reference to FIG. 25(b).

The communication node may perform CCA during a CCA slot. If the channelis unoccupied in a first CCA slot, the communication node may transmitdata by securing a time period of up to (13/32)q ms.

On the contrary, if the channel is occupied in the first CCA slot, thecommunication node selects N (Nϵ{1, 2, . . . , q}) arbitrarily (i.e.,randomly) and stores the selected N value as an initial count. Then, thecommunication node senses a channel state on a CCA slot basis. Each timethe channel is unoccupied in one specific CCA slot, the communicationnode decrements the count by 1. If the count is 0, the communicationnode may transmit data by securing a time period of up to (13/32)q ms.

3.3 Discontinuous Transmission in DL

When discontinuous transmission is performed on an unlicensed carrierhaving a limited maximum transmission period, the discontinuoustransmission may influence on several functions necessary for performingan operation of LTE system. The several functions can be supported byone or more signals transmitted at a starting part of discontinuous LAADL transmission. The functions supported by the signals include such afunction as AGC configuration, channel reservation, and the like.

When a signal is transmitted by an LAA node, channel reservation has ameaning of transmitting signals via channels, which are occupied totransmit a signal to other nodes, after channel access is performed viaa successful LBT operation.

The functions, which are supported by one or more signals necessary forperforming an LAA operation including discontinuous DL transmission,include a function for detecting LAA DL transmission transmitted by a UEand a function for synchronizing frequency and time. In this case, therequirement of the functions does not mean that other availablefunctions are excluded. The functions can be supported by other methods.

3.3.1 Time and Frequency Synchronization

A design target recommended by LAA system is to support a UE to make theUE obtain time and frequency synchronization via a discovery signal formeasuring RRM (radio resource management) and each of reference signalsincluded in DL transmission bursts, or a combination thereof. Thediscovery signal for measuring RRM transmitted from a serving cell canbe used for obtaining coarse time or frequency synchronization.

3.3.2 DL Transmission Timing

When a DL LAA is designed, it may follow a CA timing relation betweenserving cells combined by CA, which is defined in LTE-A system (Rel-12or earlier), for subframe boundary adjustment. Yet, it does not meanthat a base station starts DL transmission only at a subframe boundary.Although all OFDM symbols are unavailable in a subframe, LAA system cansupport PDSCH transmission according to a result of an LBT operation. Inthis case, it is required to support transmission of control informationnecessary for performing the PDSCH transmission.

3.4 Measuring and Reporting RRM

LTE-A system can transmit a discovery signal at a start point forsupporting RRM functions including a function for detecting a cell. Inthis case, the discovery signal can be referred to as a discoveryreference signal (DRS). In order to support the RRM functions for LAA,the discovery signal of the LTE-A system and transmission/receptionfunctions of the discovery signal can be applied in a manner of beingchanged.

3.4.1 Discovery Reference Signal (DRS)

A DRS of LTE-A system is designed to support on/off operations of asmall cell. In this case, off small cells correspond to a state thatmost of functions are turned off except a periodic transmission of aDRS. DRSs are transmitted at a DRS transmission occasion with a periodof 40, 80, or 160 ms. A DMTC (discovery measurement timingconfiguration) corresponds to a time period capable of anticipating aDRS received by a UE. The DRS transmission occasion may occur at anypoint in the DMTC. A UE can anticipate that a DRS is continuouslytransmitted from a cell allocated to the UE with a correspondinginterval.

If a DRS of LTE-A system is used in LAA system, it may bring newconstraints. For example, although transmission of a DRS such as a veryshort control transmission without LBT can be permitted in severalregions, a short control transmission without LBT is not permitted inother several regions. Hence, a DRS transmission in the LAA system maybecome a target of LBT.

When a DRS is transmitted, if LBT is applied to the DRS, similar to aDRS transmitted in LTE-A system, the DRS may not be transmitted by aperiodic scheme. In particular, it may consider two schemes described inthe following to transmit a DRS in the LAA system.

As a first scheme, a DRS is transmitted at a fixed position only in aDMTC configured on the basis of a condition of LBT.

As a second scheme, a DRS transmission is permitted at one or moredifferent time positions in a DMTC configured on the basis of acondition of LBT.

As a different aspect of the second scheme, the number of time positionscan be restricted to one time position in a subframe. If it is moreprofitable, DRS transmission can be permitted at the outside of aconfigured DMTC as well as DRS transmission performed in the DMTC.

FIG. 26 is a diagram for explaining DRS transmission methods supportedby LAA system.

Referring to FIG. 26, the upper part of FIG. 26 shows the aforementionedfirst scheme for transmitting a DRS and the bottom part of FIG. 10 showsthe aforementioned second scheme for transmitting a DRS. In particular,in case of the first scheme, a UE can receive a DRS at a positiondetermined in a DMTC period only. On the contrary, in case of the secondscheme, a UE can receive a DRS at a random position in a DMTC period.

In LTE-A system, when a UE performs RRM measurement based on DRStransmission, the UE can perform single RRM measurement based on aplurality of DRS occasions. In case of using a DRS in LAA system, due tothe constraint of LBT, it is difficult to guarantee that the DRS istransmitted at a specific position. Even though a DRS is not actuallytransmitted from a base station, if a UE assumes that the DRS exists,quality of an RRM measurement result reported by the UE can bedeteriorated. Hence, when LAA DRS is designed, it is necessary to permitthe existence of a DRS to be detected in a single DRS occasion. By doingso, it may be able to make the UE combine the existence of the DRS withRRM measurement, which is performed on successfully detected DRSoccasions only.

Signals including a DRS do not guarantee DRS transmissions adjacent intime. In particular, if there is no data transmission in subframesaccompanied with a DRS, there may exist OFDM symbols in which a physicalsignal is not transmitted. While operating in an unlicensed band, othernodes may sense that a corresponding channel is in an idle state duringa silence period between DRS transmissions. In order to avoid theabovementioned problem, it is preferable that transmission burstsincluding a DRS signal are configured by adjacent OFDM symbols in whichseveral signals are transmitted.

3.5 Channel Access Procedure and Contention Window Adjustment Procedure

In the following, the aforementioned channel access procedure and thecontention window adjustment procedure are explained in the aspect of atransmission node.

FIG. 27 is a flowchart for explaining CAP and CWA.

In order for an LTE transmission node (e.g., a base station) to operatein LAA Scell(s) corresponding to an unlicensed band cell for DLtransmission, it may initiate a channel access procedure (CAP) [S2710].

The base station can randomly select a back-off counter N in acontention window (CW). In this case, the N is configured by an initialvalue Ninit [S2720]. The Ninit is randomly selected from among valuesranging from 0 to CW_(p).

Subsequently, if the back-off counter value (N) corresponds to 0[S2722], the base station terminates the CAP and performs Tx bursttransmission including PSCH [S2724]. On the contrary, if the back-offvalue is not 0, the base station reduces the back-off counter value by 1[S2730].

The base station checks whether or not a channel of the LAA Scell(s) isin an idle state. If the channel is in the idle state, the base stationchecks whether or not the back-off value corresponds to 0 [S2750]. Thebase station repeatedly checks whether or not the channel is in the idlestate until the back-off value becomes 0 while reducing the back-offcounter value by 1.

In the step S2740, if the channel is not in the idle state i.e., if thechannel is in a busy state, the base station checks whether or not thechannel is in the idle state during a defer duration (more than 15 usec)longer than a slot duration (e.g., 9 usec) [S2742]. If the channel is inthe idle state during the defer duration, the base station can resumethe CAP [S2744]. For example, when the back-off counter value Ninitcorresponds to 10, if the channel state is determined as busy after theback-off counter value is reduced to 5, the base station senses thechannel during the defer duration and determines whether or not thechannel is in the idle state. In this case, if the channel is in theidle state during the defer duration, the base station performs the CAPagain from the back-off counter value 5 (or, from the back-off countervalue 4 by reducing the value by 1) rather than configures the back-offcounter value Ninit. On the contrary, if the channel is in the busystate during the defer duration, the base station performs the stepS2742 again to check whether or not the channel is in the idle stateduring a new defer duration.

Referring back to FIG. 27, the base station checks whether or not theback-off counter value (N) becomes 0 [S2750]. If the back-off countervalue (N) becomes 0, the base station terminates the CAP and may be ableto transmit a Tx burst including PDSCH.

The base station can receive HARQ-ACK information from a UE in responseto the Tx burst [S2770]. The base station can adjust a CWS (contentionwindow size) based on the HARQ-ACK information received from the UE[S2780].

In the step S2780, as a method of adjusting the CWS, the base stationcan adjust the CWS based on HARQ-ACK information on a first subframe ofa most recently transmitted Tx burst (i.e., a start subframe of the Txburst).

In this case, the base station can set an initial CW to each priorityclass before the CWP is performed. Subsequently, if a probability thatHARQ-ACK values corresponding to PDSCH transmitted in a referencesubframe are determined as NACK is equal to or greater than 80%, thebase station increases CW values set to each priority class to a nexthigher priority.

In the step S2760, PDSCH can be assigned by a self-carrier schedulingscheme or a cross-carrier scheduling scheme. If the PDSCH is assigned bythe self-carrier scheduling scheme, the base station counts DTX,NACK/DTX, or ANY state among the HARQ-ACK information fed back by the UEas NACK. If the PDSCH is assigned by the cross-carrier schedulingscheme, the base station counts the NACK/DTX and the ANY states as NACKand does not count the DTX state as NACK among the HARQ-ACK informationfed back by the UE.

If bundling is performed over M (M>=2) number of subframes and bundledHARQ-ACK information is received, the base station may consider thebundled HARQ-ACK information as M number of HARQ-ACK responses. In thiscase, it is preferable that a reference subframe is included in the Mnumber of bundled subframes.

4. Proposed Embodiments

4.1 Method of Transmitting and Receiving Tracking Reference Signal inLAA System

FIGS. 28 to 30 are diagrams illustrating a method of transmitting andreceiving a TRS (tracking reference signal) according to a proposedembodiment.

A base station can perform LBT to transmit data or discontinuouslytransmit data for a different reason on an unlicensed band. Thediscontinuous data transmission influences on reception capability of adata channel (e.g., PDSCH) and/or a control channel (e.g., (E)PDCCH) ofa user equipment. In particular, in order for the user equipment toreceive a specific downlink channel, it is necessary for the userequipment to configure a coefficient of a filter for estimating achannel based on a result of estimating such a parameter as power delayprofile, delay spread and Doppler spectrum, Doppler spread, etc. And, itis necessary for the user equipment to prepare synchronization andtracking based on a result of estimating a time/frequency error. Theuser equipment can successfully receive a downlink channel only when thepreparation is completed using the abovementioned parameters. When achannel of an unlicensed band is in an idle state at specific timing,although a base station performs an LBT procedure and transmits adownlink channel to a user equipment, if a certain work for receivingthe downlink channel is not preferentially performed, the user equipmentmay fail to receive the downlink channel.

Hence, synchronization and tracking requirements for receiving adownlink channel can be defined for a user equipment. For example, ifthe number of tracking reference signals (TRSs) received during aspecific time period or a time window is equal to or greater than aprescribed number, a base station and a user equipment can define it asa requirement for tracking performance of the user equipment issatisfied. The requirement can be randomly configured. The requirementcan be differently determined according to a modulation order of dataincluded in a downlink channel to be received by the user equipment.

In the following embodiment, when a tracking requirement is defined, amethod of transmitting and receiving a TRS for satisfying therequirement is proposed.

As mentioned in the foregoing description, in a legacy LTE/LTE-A system,a DRS is utilized to discover a cell which is deactivated due to no datatraffic. The DRS is configured by a CSI-RS (optional) in addition to aPSS/SSS and a CRS. A base station periodically sets DMTC to a userequipment in a unit of 6 ms and transmits a DRS within the DMTC. Theuser equipment identifies a cell and performs RRM by receiving the DRSwithin the set DMTC. Meanwhile, according to the proposed embodiment,the base station and the user equipment can utilize the DRS to performtracking. In particular, the base station and the user equipment utilizethe DRS as a TRS to satisfy an insufficient tracking requirement.

Meanwhile, when data to be transmitted in a manner of being offloaded toan unlicensed band are intensively arrived, if a DMTC section isconfigured too often, although there is no data to be transmitted, sincea DRS is frequently transmitted, it could be inefficient. Moreover, ifthe DMTC section is frequently configured, interference affecting adifferent communication system (e.g., WiFi system or inter-operator LAAsystem) coexisted on an unlicensed band increases. On the contrary, ifthe DMTC section is configured to be too long, since a time section fortransmitting a DRS is reduced, RSs for performing synchronization andtracking become insufficient. As a result, a probability of failing tosatisfy a requirement for tracking performance increases.

In order to solve the abovementioned problem, as shown in FIG. 28, abase station can configure DMTC of two types. For example, a primaryDMTC corresponds to a time section having a relatively long period and asecondary DMTC corresponds to a time section having a relatively shortperiod. In particular, in FIG. 28, such a relation as T1>T2 issatisfied. According to one embodiment, a base station always performsLBT during the primary DMTC to transmit a DRS. If it is determined thatit is necessary to transmit a DRS (e.g., if it is determined that atracking requirement is not satisfied), the base station selectivelyperforms LBT during the secondary DMTC to transmit the DRS. If it isdetermined that a tracking requirement required by a user equipment issatisfied, the base station may omit transmission of a DRS during thesecondary DMTC [2810]. In particular, DRS transmission via the secondaryDMTC can be configured in advance or can be restrictively configuredonly when the DRS transmission is necessary. The base station canexplicitly inform the user equipment of whether or not a DRS istransmitted via the secondary DMTC using DCI.

The primary DMTC can be set to the base station using a scheme identicalor similar to a legacy LTE/LTE-A system. Meanwhile, the base station canset the secondary DMTC to the user equipment via higher layer signaling.The base station can set both the primary DMTC and the secondary DMTC tothe user equipment. And, the base station may commonly set the secondaryDMTC to user equipments belonging to a cell. Moreover, a section wherethe primary DMTC and the secondary DMTC are overlapped can be regardedas the primary DMTC. A procedure of measuring a neighboring cell can beperformed during the primary DMTC only.

FIGS. 29 and 30 are flowcharts illustrating a different embodiment for amethod of transmitting and receiving a TRS. FIG. 29 illustrates aprocedure of transmitting a TRS in the aspect of a base station and FIG.30 illustrates a procedure of receiving a TRS in the aspect of a userequipment, respectively.

First of all, the base station can utilize other signals rather than aDRS as a TRS. This is because it is not easy to satisfy a trackingrequirement with a DRS transmitted in a unit of scores of ms inLTE/LTE-A system. For example, a DMRS or a preamble indicating the startof downlink transmission can be utilized as a TRS. In particular, inaddition to a DRS (PSS, SSS, CSI-RS), a DMRS and a DL preamble can beutilized as a TRS. The base station transmits the DMRS and the DLpreamble to the user equipment to achieve a requirement satisfyingtracking performance.

Specifically, the base station determines that a tracking performancerequirement of the user equipment intending to transmit downlink data isnot satisfied [S2910]. If tracking performance is not sufficient, it isnecessary for the base station to preferentially transmit a TRS to theuser equipment before downlink data is transmitted to the userequipment. Hence, the base station transmits the TRS to the userequipment in a subframe in which a DRS is not transmitted. Meanwhile,since the user equipment may be unaware of a fact that the base stationtransmits a DMRS and a DL preamble as a TRS, the base station informsthe user equipment of the transmission of a TRS via UE-specific DCI inadvance [S2920]. The DCI can be transmitted to the user equipment via aPcell and/or UScell (Scell of unlicensed band). For example, the basestation changes at least one selected from the group consisting of ascrambling sequence of the DCI, a CRS mask, and a search space or adds anew indicator to the DCI to inform the user equipment that the TRS istransmitted in a corresponding subframe. In particular, a scheme ofinforming each user equipment of TRS transmission has a merit in that itis able to reuse a DCI format compared to a scheme of commonly informingall user equipments belonging to cell coverage of TRS transmission viacommon DCI of Pcell. On the contrary, if there are many user equipmentsrequiring a TRS, it is more efficient to use a scheme that the basestation commonly assigns an RNTI value to user equipments connected withthe base station and informs the user equipments of transmission of aTRS using the RNTI value.

Having received a signal indicating transmission of a TRS from the basestation, the user equipment performs tracking using the received TRS ina corresponding subframe. In particular, the user equipment utilizes notonly a DRS received in the subframe but also a DMRS and/or a DL preambleas the TRS [S2930]. As a result, although it is not sufficient with theDRS only, the user equipment can satisfy a tracking requirement.

Unlike the aforementioned embodiment, the user equipment mayautonomously determine that the tracking requirement is not satisfied.If the user equipment not satisfied with the tracking requirementreceives DCI indicating transmission of a TRS from the base station[S2920], the user equipment does not attempt to demodulate PDSCHscheduled in a corresponding subframe and utilizes the subframe for atracking use only [S2930]. In this case, the user equipment may utilizea scheduled RB of the subframe or RSs belonging to the entire RBs asTRSs. If the tracking requirement is not satisfied, although downlinkdata transmission is scheduled, the user equipment does not transmitHARQ-ACK/NACK to the base station and the base station does not expectHARQ-ACK/NACK from the user equipment.

Meanwhile, although the base station transmits TRSs to the userequipment as many as TRSs satisfying tracking performance of the userequipment, the user equipment may fails to receive a part of RSstransmitted by the base station. For example, if there exists a hiddennode near the user equipment, the user equipment may determine that atracking requirement is not satisfied despite of the transmission of thebase station. In this case, a mismatch problem that the number of TRSstransmitted to the user equipment in the aspect of the base station isnot matched with the number of TRSs received from the base station inthe aspect of the user equipment may occur. As a result, trackingperformance of the user equipment is not sufficiently satisfied [S3010].

Since the base station does not know the status of the user equipment,the base station determines that the tracking performance of the userequipment is satisfied and schedules general downlink data rather thantransmits a TRS [S3020]. Yet, since the tracking performance is notsatisfied, the user equipment fails to successfully receive the downlinkdata and utilizes a subframe as a tracking use only [S3030]. Or,irrespective of whether or not a tracking requirement is satisfied inthe aspect of transmission of the base station, if a trackingrequirement is not guaranteed in the aspect of reception of the userequipment, the same result may occur. In particular, if a trackingrequirement is not satisfied in the aspect of reception, the userequipment utilizes a corresponding subframe for a tracking use onlyrather than processes downlink data.

In this case, if PDSCH scheduled in a received subframe corresponds to afirstly transmitted data, the user equipment may not feedback HARQACK/NACK to the base station [S3040]. Or, the user equipment mayfeedback ‘DTX’ to the base station to initialize an RV (redundancyversion) value. Or, the user equipment may not perform buffering on thereceived data.

If PDSCH scheduled in the received subframe corresponds to aretransmitted data, the user equipment may not feedback HARQ ACK/NACK tothe base station or feedback ‘DTX’ to the base station to maintain an RVvalue. Or, the user equipment may feedback NACK to the base station toprevent an aggregation level from being excessively higher [S3050]. Or,the user equipment may not perform combining between data received in aprevious subframe and data previously received.

Meanwhile, a user equipment not satisfied with a TRS requirement candetect not only PDSCH scheduled to the user equipment but also PDSCHtransmitted to a different user equipment via blind detection. Inparticular, the user equipment may utilize an RS or a DL preambleincluded in PDSCH of a neighboring user equipment for a trackingprocedure of the user equipment [S3060].

Meanwhile, if a user equipment determines that a tracking requirement isnot satisfied, the user equipment may directly ask a base station totransmit a TRS [S3070]. For example, the user equipment can periodicallyreceive a PUCCH resource for requesting TRS transmission from the basestation via a procedure similar to a scheduling request. If a TRS isadditionally required, the user equipment may ask the base station totransmit a TRS using a resource allocated in advance. As a differentexample, if a TRS transmission request is triggered by receiving DCIfrom the base station, the user equipment may inform the base station ofwhether or not TRS transmission is necessary via a resource mapped tothe DCI.

In particular, the user equipment can satisfy a tracking requirement ofthe user equipment via various methods. The abovementioned embodimentscan be implemented independently or in a manner of being combined with adifferent example. Moreover, the embodiments are just examples forsatisfying a tracking requirement of the user equipment. Unmentionedvarious examples can be utilized for transmitting and receiving a TRSand achieving a tracking requirement. And, the base station can informthe user equipment of information on whether or not the embodiments areapplied and detail information on a parameter and a rule via physicallayer signaling or higher layer signal.

4.2 RRM Measurement Method in LAA System

FIG. 31 is a flowchart for a method of performing RRM (radio resourcemeasurement) according to a proposed embodiment.

In small cell environment of a legacy LTE system, a DRS is utilized forperforming RRM and is transmitted to a user equipment within a DMTCsection of 6 ms unit to discover a cell which is deactivated (or, turnedoff) due to no traffic. The user equipment performs cell identification,RRM, and the like using the DRS received within the DMTC section.

Meanwhile, since LBT is performed on an unlicensed band of LAA system,although a serving cell is activated, it is difficult to alwaysguarantee downlink transmission in the serving cell. In particular, asmentioned earlier in the paragraph 3.4, the LAA system basicallysupports RRM measurement via a DRS. Yet, since a base station performsan LBT operation not only for transmission of downlink data but also fortransmission of the DRS, it may be difficult to always guarantee thetransmission of the DRS for performing RRM.

In the following, when the transmission of the DRS is not sufficientlyguaranteed due to the LBT, a method of performing RRM and a method ofreporting the RRM are proposed.

Basically, a base station sets a period of reporting an RRM measurementresult to a user equipment [S3110]. The user equipment performs RRMmeasurement using samples capable of performing the RRM measurementduring the period set to the user equipment, applies L1/L3 filtering toa measurement result, and reports the final result value to the basestation.

In this case, if the base station fails to perform LBT, the base stationmay not transmit a DRS to the user equipment. Or, a DRS transmitted bythe base station may not be successfully received by the user equipment.In this case, the user equipment determines that measurement samplesobtained during a prescribed period are not sufficient [S3120]. If theminimum number of measurement samples required for reporting such ameasurement result as RSRP, RSSI, RSRQ, and the like is defined, an RRMresult measured by the insufficient measurement samples is incorrect.

In the following, a proposed embodiment for a procedure of measuring andreporting RRM using the insufficient number of samples is explained.First of all, if a user equipment performs RRM measurement usinginsufficient samples, the user equipment may not report a result of themeasurement to a base station. On the contrary, when the user equipmentreports a measurement result to the base station, the user equipment canalso inform the base station that the measurement is performed viainsufficient samples [S3130]. For example, the user equipment can informthe base station of a measurement result together with a fact that themeasurement result is measured from insufficient samples. Or, the userequipment may inform the base station of only information indicatingthat the samples were not sufficient using a predefined value. Or, theuser equipment may report a measurement result calculated via adifferent method (e.g., a mode value, a median value, or a maximumvalue) to the base station without applying L1/L3 filtering. In case ofreporting a measurement result measured using a different method insteadof the L1/L3 filtering, the user equipment can also inform the basestation that the measurement is performed via insufficient samples.

Meanwhile, irrespective of a case that the base station fails totransmit the sufficient number of DRSs, although the base stationtransmits the sufficient number of DRSs to the user equipment, the userequipment may fail to receive the sufficient number of DRSs transmittedby the base station due to a hidden node problem and the like. In thiscase, a mismatch problem occurs between timing capable of performing RRMmeasurement in the aspect of the base station and timing capable ofperforming RRM measurement in the aspect of the user equipment.

In order to solve the mismatch problem, when the user equipment reportsan RRM measurement result to the base station, the user equipment canalso report time information (e.g., timestamp information or loginformation) indicating the timing at which the measurement is performedto the base station. Yet, if the user equipment informs the base stationof the time information together with the measurement result, it maybecome signaling overhead. And, since all user equipments belonging to aserving cell perform measurement and report a measurement result, it maylead to a result that excessive resources are required for reporting themeasurement result.

In order to prevent occurrence of a problem while the aforementionedmismatch problem is solved, the user equipment can additionally reportvalues described in the following to the base station in addition to aresult value to which the L1/L3 filtering is applied. For example, theuser equipment can inform the base station of information indicating thenumber of samples (e.g., the number of DRSs, the number of subframes,the number of OFDM symbols, etc.) on which measurement is performed. Or,the user equipment can inform the base station of information on atleast one selected from the group consisting of a maximum value, aminimum value, a mode value, and a median value among values measuredduring a period of reporting a measurement result.

In a situation that the sufficient number of samples required forperforming RRM measurement is not secured, the user equipment can informthe base station that a measurement result is incorrect by forwardingadditional information to the base station. The abovementionedembodiments can be implemented independently or in a manner of beingcombined with a different example. Moreover, the embodiments are justexamples for performing measurement and reporting a measurement result.Unmentioned various examples can be utilized in a similar form. And, thebase station can inform the user equipment of information on whether ornot the embodiments are applied and detail information on a parameterand a rule via physical layer signaling or higher layer signal.

5. Device Configuration

FIG. 32 is a diagram illustrating configurations of a UE and an eNBcapable of being implemented by the embodiments proposed in the presentinvention. The UE and the eNB shown in FIG. 32 operate to implementembodiments of a method of transmitting and receiving a trackingreference signal and a method of performing RRM measurement.

A UE may act as a transmission end on a UL and as a reception end on aDL. An eNB may act as a reception end on a UL and as a transmission endon a DL.

That is, each of the UE and the eNB may include a Transmitter (Tx) 3240or 3250 and a Receiver (Rx) 3260 or 3270, for controlling transmissionand reception of information, data, and/or messages, and an antenna 3200or 3210 for transmitting and receiving information, data, and/ormessages.

Each of the UE and the eNB may further include a processor 32820 or 3230for implementing the afore-described embodiments of the presentdisclosure and a memory 3280 or 3290 for temporarily or permanentlystoring operations of the processor 32820 or 3230.

The Tx and Rx of the UE and the eNB may perform a packetmodulation/demodulation function for data transmission, a high-speedpacket channel coding function, OFDM packet scheduling, TDD packetscheduling, and/or channelization. Each of the UE and the eNB of FIG. 32may further include a low-power Radio Frequency (RF)/IntermediateFrequency (IF) module.

Meanwhile, the UE may be any of a Personal Digital Assistant (PDA), acellular phone, a Personal Communication Service (PCS) phone, a GlobalSystem for Mobile (GSM) phone, a Wideband Code Division Multiple Access(WCDMA) phone, a Mobile Broadband System (MBS) phone, a hand-held PC, alaptop PC, a smart phone, a Multi Mode-Multi Band (MM-MB) terminal, etc.

The smart phone is a terminal taking the advantages of both a mobilephone and a PDA. It incorporates the functions of a PDA, that is,scheduling and data communications such as fax transmission andreception and Internet connection into a mobile phone. The MB-MMterminal refers to a terminal which has a multi-modem chip built thereinand which can operate in any of a mobile Internet system and othermobile communication systems (e.g. CDMA 2000, WCDMA, etc.).

Embodiments of the present disclosure may be achieved by various means,for example, hardware, firmware, software, or a combination thereof.

In a hardware configuration, the methods according to exemplaryembodiments of the present disclosure may be achieved by one or moreApplication Specific Integrated Circuits (ASICs), Digital SignalProcessors (DSPs), Digital Signal Processing Devices (DSPDs),Programmable Logic Devices (PLDs), Field Programmable Gate Arrays(FPGAs), processors, controllers, microcontrollers, microprocessors,etc.

In a firmware or software configuration, the methods according to theembodiments of the present disclosure may be implemented in the form ofa module, a procedure, a function, etc. performing the above-describedfunctions or operations. A software code may be stored in the memory3280 or 3290 and executed by the processor 3220 or 3230. The memory islocated at the interior or exterior of the processor and may transmitand receive data to and from the processor via various known means.

Those skilled in the art will appreciate that the present disclosure maybe carried out in other specific ways than those set forth hereinwithout departing from the spirit and essential characteristics of thepresent disclosure. The above embodiments are therefore to be construedin all aspects as illustrative and not restrictive. The scope of thedisclosure should be determined by the appended claims and their legalequivalents, not by the above description, and all changes coming withinthe meaning and equivalency range of the appended claims are intended tobe embraced therein. It is obvious to those skilled in the art thatclaims that are not explicitly cited in each other in the appendedclaims may be presented in combination as an embodiment of the presentdisclosure or included as a new claim by a subsequent amendment afterthe application is filed.

INDUSTRIAL APPLICABILITY

The present disclosure is applicable to various wireless access systemsincluding a 3GPP system, and/or a 3GPP2 system. Besides these wirelessaccess systems, the embodiments of the present disclosure are applicableto all technical fields in which the wireless access systems find theirapplications. Moreover, the proposed method can also be applied tommWave communication using an ultra-high frequency band.

What is claimed is:
 1. A method of receiving a TRS (tracking referencesignal), which is received by a user equipment in a wirelesscommunication system supporting an unlicensed band, the methodcomprising: receiving, from a base station, information indicating thata TRS is transmitted in a subframe; receiving the TRS transmitted in thesubframe; and performing tracking using the TRS, wherein the userequipment does not demodulate data or does not transmit ACK/NACK(acknowledgment/negative ACK) in the subframe in which the TRS istransmitted.
 2. The method of claim 1, wherein the subframe in which theTRS is transmitted corresponds to a subframe other than a subframe inwhich a DRS (discovery reference signal) is transmitted.
 3. The methodof claim 1, wherein the TRS is composed of at least one selected fromthe group consisting of a DRS, a DMRS (demodulation reference signal),and a downlink transmission preamble.
 4. The method of claim 1, whereinthe information indicating that the TRS is transmitted comprisesUE-specifically transmitted DCI (downlink control information) or anRNTI (radio network temporary identifier) commonly assigned to userequipments connected with the base station.
 5. The method of claim 1,wherein the tracking is performed using TRSs equal to or greater thanthe prescribed number of TRSs satisfying a tracking requirementconfigured by the base station and is performed using both a TRSreceived within a time section configured by the base station and a TRSreceived in the subframe.
 6. The method of claim 1, when the userequipment determines that a tracking requirement for performing trackingis not satisfied, further comprising requesting of TRS transmission tothe base station.
 7. The method of claim 1, wherein when a trackingrequirement is not satisfied, the subframe comprises a secondary DMTC(discovery measurement timing configuration) configured with a periodshorter than a period of a primary DMTC.
 8. A user equipment receiving aTRS (tracking reference signal) in a wireless communication systemsupporting an unlicensed band, the user equipment comprising: atransmitter; a receiver; and a processor configured to operate in amanner of being connected with the transmitter and the receiver, theprocessor receives, from a base station, information indicating that aTRS is transmitted in a subframe, the processor receives the TRStransmitted in the subframe, the processor performs tracking using theTRS, wherein the user equipment does not demodulate data or does nottransmit ACK/NACK (acknowledgment/negative ACK) in the subframe in whichthe TRS is transmitted.
 9. The user equipment of claim 8, wherein thesubframe in which the TRS is transmitted corresponds to a subframe otherthan a subframe in which a DRS (discovery reference signal) istransmitted.
 10. The user equipment of claim 8, wherein the TRS iscomposed of at least one selected from the group consisting of a DRS, aDMRS (demodulation reference signal), and a downlink transmissionpreamble.
 11. The user equipment of claim 8, wherein the informationindicating that the TRS is transmitted comprises UE-specificallytransmitted DCI (downlink control information) or an RNTI (radio networktemporary identifier) commonly assigned to user equipments connectedwith the base station.
 12. The user equipment of claim 8, wherein thetracking is performed using TRSs equal to or greater than the prescribednumber of TRSs satisfying a tracking requirement configured by the basestation and is performed using both a TRS received within a time sectionconfigured by the base station and a TRS received in the subframe. 13.The user equipment of claim 8, wherein when the user equipmentdetermines that a tracking requirement for performing tracking is notsatisfied, the processor requests TRS transmission to the base station.14. The user equipment of claim 8, wherein when a tracking requirementis not satisfied, the subframe comprises a secondary DMTC (discoverymeasurement timing configuration) configured with a period shorter thana period of a primary DMTC.